There are two fixed oxygen systems, one for the cockpit and one for the cabin.  Oxygen for both systems. Cockpit and cabin, is provided by high pressure cylinders. 
Three cylinders are located along the right wall of the electronic equipment compartment.  The forward cylinder provides oxygen to the cockpit.  The other two cylinders provide oxygen to the cabin system. 

 Each cylinder has its own shutoff valves, pressure gauge and overpressure relief. 

The high pressure of the oxygen from the crew cylinder is lowered by a reducer/regulator.  Lower pressure then flows to the cockpit shutoff valve.  Oxygen from the reducer/regulator flows to each individual regulator and mask in the cockpit. 

Minimum crew oxygen pressure for dispatch is normally around 1200 psi, 1450 psi for the passenger oxygen system.

System Components

Crew Oxygen Cylinder Assembly
The crew oxygen cylinder assembly consists of an oxygen cylinder charged with 114 cubic feet of standard oxygen compressed to 1850 psig -at 70'F, a slow opening shutoff valve, a pressure gauge, and a safety discharge disc which blows out before pressure reaches a value which could damage tubing or components. The safety discharge disc is connected to an overboard discharge line which runs to an oxygen discharge indicator at the airplane skin. The discharge line is connected also to the passenger oxygen cylinders. Incorporated into the discharge indicator is a plastic disc which normally prevents dirt, grease, etc. from entering the discharge line, but which blown out at a pressure of 2650 - 2950 psig to allow oxygen to discharge overboard. The oxygen cylinder assembly is mounted on the right hand side of the airplane immediately forward of the forward cargo compartment in the E & E bay. 

Crew Oxygen Shutoff Valve
The crew system shutoff valve is a manually operated valve that shuts off oxygen pressure to the flight crew oxygen system. The valve in located in the control cabin, Fwd sidewall Front end of the engineers panel.

Crew Oxygen Pressure Reducing Regulator
The pressure-reducing regulator in mounted adjacent to the oxygen cylinder. It consists of inlet and outlet ports, a valve, an actuator shaft, an actuator linkage, a diaphragm and a spring. Operating pressures of the regulator are as follows:
Flow will take place with an inlet pressure of 50 to 1850 psig. Outlet pressure is a variable depending on inlet pressure and flow rate (in litres per minute).
Outlet pressure will always be in the range of 50 to 75 psig.
"Regulated pressure " is outlet pressure at zero flow and a specified inlet pressure Regulated pressure for this regulator is set at 70 to 75 psig when inlet pressure is 150 psig.
The normal operating cycle is as follows:
Oxygen flows through the outlet port to the diluter demand regulator lowering the pressure inside the pressure-reducing regulator. This allows the spring to overcome the diaphragm force and push the actuator linkage down, which causes the actuator shaft to open the inlet valve admitting more oxygen into the pressure-reducing regulator to continue the cycle as necessary to maintain the outlet pressure within the specified range.
The relief valve is set to open at pressures, in excess of 100 to 110 psig, and reset at 90 to 100 psig.

Crew Oxygen Distribution Line and Overboard Discharge Line
The oxygen distribution line has a high-pressure section and a low-pressure section. The high-pressure section, between the oxygen cylinder assembly and the pressure-reducing regulator is made of stainless steel tubing. The low-pressure section, downstream of the regulator, is made of aluminium alloy tubing. Flareless tube fittings are used throughout the system. The overboard discharge line connects the oxygen cylinder safety discharge disc. This line is made of stainless steel tubing.
In the event any cylinder overpressure, a valve relieves all of that bottle's oxygen.  It will be dumped overboard through the overpressure discharge line, common to all three cylinders.  The overboard discharge port is located on the right side of the fuselage just below the E & E cooling exhaust and is sealed with a green disc.  The overpressure discharge lines from all three cylinders vent through this one port, a missing green disc indicates a possible thermal overpressure discharge from one or more cylinders.  If the disc is missing or broken, relief may have taken place and further investigation is required. 

Diluter Demand Regulator
A diluter demand regulator in mounted at each flight crew station. Each regulator face displays a float-type flow indicator, which signals oxygen now through the regulator to the mask. The regulator face also displays three manual control levers. A supply lever ('.SUPPLY ON-OFF") is used to open or close the oxygen supply valve. An emergency lever ("EMERGENCY ON-OFF") is used to obtain oxygen under pressure. An oxygen selection lever ("OXYGEN 100%-NORMAL") in used for selecting the air / oxygen mixture or oxygen only. Operation of the regulator in as follows: With the supply lever in the "ON" position, the oxygen selection lever in the "NORMAL" position and the emergency lever in the "OFF" position, oxygen enters the regulator inlet. When there is sufficient differential pressure across the demand diaphragm.
The demand valve opens to supply oxygen to the mask. This pressure differential exists during the user's inhalation cycle. After passing through the demand valve the oxygen in mixed with air that enters through the air inlet port. The mixture ratio is determined by an aneroid controlled metering valve which provides a high oxygen ratio at high cabin altitudes, and a high air ratio at lower cabin altitudes. An air inlet valve is set to permit the air flow to begin at the same time as the oxygen flow. The additional air may be cut off by turning the oxygen selection lever to "OXYGEN 100%". When this lever in "OXYGEN NORMAL" air enters through the air inlet port and the required amount is added to the oxygen to form the correct air/oxygen mixture. Positive pressure at the regulator outlet may be obtained by operating the emergency lever to "ON". This mechanically loads the demand diaphragm to provide positive outlet pressure of 2 - 4 inches of water.

External Charging Connection
External charging of the crew oxygen cylinder in accomplished through a recharging panel installed under a latched door on the right side of the lower fuselage section below the second passenger cabin window, but use is prohibited with a lot of operators. The recharging panel is common to both the crew and passenger oxygen systems. It incorporates a filler valve. Two shutoff valves and two pressure indicators. These components are accessible through the same door which provides access to the oxygen cylinder assemblies and the filler valve is common to both systems

Crew Oxygen Pressure Transducer and Indicators
The pressure transducer in mounted on the crew oxygen line near the cylinder. It contains a bourdon tube with a wiper blade. Variation in pressure repositions the wiper blade on a rheostat. A voltage regulatory circuit supplies constant voltage to the potentiometer. The circuit in completed with two pressure indicators one on the external recharging panel and one on the third crewman's panel. Each indicator consists of a unit which measures a voltage output signal from the pressure transducer and indicates the corresponding pressure on a dial calibrated from 0 to 2000 psi.

Thermal Compensator Assemblies
Thermal compensator assemblies are installed at various places in the system. Each acts as a heat sink and tends to prevent excessive temperature build-up when the tubing in being pressurised. A thermal compensator assembly consists of a brush-like wire element. Approximately 5 inches long, inserted into a stainless steel tube. Each assembly is attached directly to an oxygen system component. The wire element fits tightly into the tube and in located in the end of the compensator assembly that attaches to the component. There are two types of thermal compensator assemblies:
The compensator assembly on the oxygen cylinder has a standard oxygen bottle fitting that attaches to the cylinder. The downstream end has a standard flareless tube connection. This compensator assembly can be handled on any flareless tube assembly. The compensator assembly on any other component, Such as a shutoff valve or regulator, is attached to the component by means of a corrosion resistant steel union. The wire element extends through the union, making it necessary to observe special precautions when removing or installing the assembly. The upstream end of the compensator assembly has a standard flareless tube connection.

Masks
The masks in the cockpit are of the "quick-donning", or "sweep-on" type. The oxygen mask mike is activated by the associated push to talk button, labelled "PTT". The oxygen/boom selector is on their respective jackboxes.  They also have a PTT (push to talk) button.  When selected to oxygen and mike selection is in PA, the associated oxygen mike is "HOT''.  The BOOM position is deactivated on some airplanes, and the oxygen position should always be selected.  Then the push to talk button or the rocker switch on the rear of each control wheel will activate the oxygen mike to what ever is selected to transmit.

Overview

 Oxygen from the two passenger cylinders flows from a common manifold into two paths.  The pressure is reduced by two parallel pressure reducing regulators and the flow is controlled by two flow valves.  Downstream of the two valves the two paths rejoin.  When either flow valve is opened, oxygen pressurises the passenger oxygen manifold, causing a pressure switch to illuminate an amber light. When cabin oxygen is required, one or both of these valves opens.  The valve can be opened on of three ways, Pneumatically, electrically or manually.  Both valves are designed to open pneumatically when cabin altitude exceeds 14,000 feet.  One opens electrically by use of the oxygen switch on the FE panel, and the other by using the manual "T" handle located on the FE aux panel below the APU control panel or under floor access panel on the 100 series. This '"T" handle can be used to close and reset both valves when oxygen is no longer needed, regardless of how the valves are activated. When the cabin oxygen manifold is pressurised. The surge of pressure trips the latches on the oxygen mask doors, and the doors open.  Four masks are then dropped at each passenger service unit and two at each flight attendant's station and in the lavatories.  Oxygen is now available to each mask valve in the cabin.   In order to establish Flow, the user must pull down the mask.  This action extracts the valve activating pin to allow flow of oxygen.  The mask delivers oxygen diluted with cabin air to the user depending on cabin altitude.  If the automatic system does not open them, the passenger doors can be opened by pushing a sharp object into a slot.  Flight attendant doors can be opened by moving a latch to the side, if the automatic system failed.  Lavatory doors are similar to the flight attendants doors.   The three ways of terminating oxygen flow are:
1. The valve can be closed by reinserting the pin into the shutoff valve assembly.
2. Pushing the manual toggles on the ends of the shutoff valves to their up position.
3. Pushing the "T" handle to reset, all the way down for 5 seconds to dump the oxygen manifold pressure, and then back to the "OFF" position.  This will close both valves. 

There is insufficient oxygen flow below 10,000' cabin altitude for passenger use.  At a cabin altitude of 10,000'  an intermittent horn will sound and can be silenced by a cut-out button on the pressurisation panel.  

Portable Oxygen Bottles
There are approximately five portable oxygen battles aboard the airplane.  Four bottles in the cabin are for passenger use, and one with a full face mask is carried in the cockpit.  Each bottle has a yellow shutoff knob and pressure gauge.