COMBUSTION: OXYGEN FIRE

The use and  handling of high pressure Oxygen can be extremely dangerous if done without knowledge; you have to be aware that it can cause fires or explosion if handled improperly.
Oxygen is not a fuel or explosive by itself, but it can decrease the ignition temperature of the materials and increase the combustion rate. To cause a fire or explosion, it is necessary to have three things: fuel, an oxidizer and heat; this is known  as the "Fire triangle" or "Combustion triangle".

In Oxygen filling all the system components must be Oxygen clean and Oxygen compatible.

OXYGEN CLEANLINESS

it's simply the absence (in the overall system) of contaminants which could act as fuel or ignition.  
The cleaning procedures are managed with chemical solutions in appropriate concentration and temperature and sometimes with specific processes (ex. ultrasonic cleaning).
It is widely accepted in the diving industry that systems with Oxygen concentration greater than 40 % are considered and managed as pure Oxygen to minimize the risk of fire; this is due to the well known "40 Percent Rule" based on an OSHA standard written for commercial diving (OSHA = Occupational Safety and Health Administration, an agency of the United States Department of Labor).

OXYGEN COMPATIBILITY

the materials are full Oxygen compatible if they are not combustible at the working pressure in presence of pure Oxygen.
A concept which is greatly misunderstood  regarding Oxygen fire hazard is that many materials used in diving rigs and filling systems are not flammable in air, but can burn in high Oxygen environments. See Ref. ASTM standard test D2863 (minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion)

Pay attention: Nitrox fires with less than 40% Oxygen have been reported, in particular with aluminum compressor parts, fill station panels and in equipment for continuous flow systems.

Also beware of lower thresholds for Oxygen enriched environments that have been established by some authoritative organizations: U.S. Navy, NASA, CGA, NFPA, ASTM,OSHA.
This means that fire hazards are real, even in systems with less than 40% Oxygen.

This part is not a disclaimer, it's a real call to proceed with caution: fire hazards are real.

The thresholds defined in the industry are not  "magic values" , they are  accepted risk limits. 

source: Swagelok Oxygen System Safety.pdf

It is interesting  to note that  the commercial diving industry has most of  the experience in brass components and 2500 psi environments (172 bar), whereas  some technical divers use aluminum 4500 psi (310 bar);  like in decompression  issues, a successful  protocol requires big databases.

Risks are not  simply related  to the Oxygen fraction and pressure, cleanliness and material compatibility. You must also consider all   the environment designs  because  fire ignition can be  triggered  in "hot spots" with local temperatures raised for a number of reasons:  system design , adiabatic compression, fluid velocity (ex.. opening valve quickly), obstructions, high flow coefficient, burrs etc.

Swagelok Excess Flow Valves

The valve is available in three sizes for different pressures and gas flow.

The block event of the valve must be fine tuned according to the booster flow, selecting the right spring for your system (available spring kit with three different springs); see XS Series Excess Flow Valve Service Instructions.pdf

These valves are available with Oxygen special cleaning according to the ASTM G93 level C: MS-06-63 (Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments).

See also: Swagelok Oxygen System Safety 

Remember:  air compressors are designed and built to run as AIR compressors, not with enriched Oxygen mix !

Follow equipment manufacturer guidelines, in terms of maximum operating conditions and maintenance.
 

Air compressors are full of small pipes  where adiabatic compression can concentrate in unpredictable "hot spots" and are full of materials that are not compatible with Oxygen: remember the fire triangle: if you want to reduce the chance of accidents when compressing Nitrox with the Air compressor, you have only one option of the triangle: keeping  the Oxygen fraction low.
 

Pay attention to homemade gas boosters; I know, Haskels are expensive ... but accidents can also be extremely expensive.

Exaggerations ?

Overconfidence can be very dangerous: search  “incident+Oxygen+fire+explosion" on the web, think about these horror stories and consider this topic again.