I am trying to assess the risk of backfilling pure oxygen into a vacuum chamber. The system is already under high vacuum, and only backfills to a pressure of 5 mbar (5.0e-0 bar.), and then sits idle for at least 30 minutes before venting to atmosphere. The inside of the chamber is heated to 300 degrees. The reason for the oxygen is to oxidise the substrates - optical glass lenses coated under vacuum with TiO2 & SiO2
Anyone have any idea where I start to even quantify this risk? Where can I find a chemical expert to help me assess this?

How big is the chamber and what is it built from? What do you mean by 'high vacuum' - are you using a simple mechanical 'high vacuum' pump or adding a vapour diffusion pump? If this is manufacturing on an industrial scale then I would guess mechanical pumping and no risks from Hg vapour in the event the system breaks? If it's a one-off then it may be using a vapour pump and you need to make sure it is fully protected against physical impact damage ...(Sorry Fred!... Yes it was my fault!)...

How is the metal applied? is it sputtered or is it vapourised?

Most likely the biggest risk will be from implosion and 'rapid unexpected dismantling' of the chamber with shrapnel of one sort or another. BUT if the kit is properly built from suitable glassware, and 'lab sized' then the risk can be adequately controlled by placing it all behind a simple perspex screen. If the Oxygen is released into the chamber too rapidly - or too much is allowed in - then the carefully applied metallic coating may begin to ball up and it will spoil the product but this is a quality rather than a safety issue. If the chamber pressure is equalised to quickly at the end of the process then there will be some cooling which may cause other issues....

But in lab bench scale apparatus - I have worked with Ultra vacuum kit - even made some of the glassware myself - and had one or two small incidents - breakage, leak - but in all honesty there is very little risk associated with it if basic care is taken.

Make sure there are no leaks on the Oxygen supply side and that you are using a suitably small cylinder - don't buy an M size cylinder if you only need a few grams (what you haven't got can't hurt you). flush the system thoroughly with Nitrogen before starting to depressurise. There will be suitable cleanliness precautions for the product that will ensure there is no flammable material in the chamber for the Oxygen to ignite - but at these sort of pressures, ignition would be highly unlikely anyway. PTFE or Viton seals do the job without lubricant... Absolutely no oil or grease anywhere in the system (or near the inlet valves) - not because of the reaction risks, but because they will boil in the vacuum, may prevent achievement of the pressure values you need, and will contaminate your product / the system. (Once had to scrap the rig and start from scratch after an accidental introduction of a few drops of lube...)

Make sure the air being allowed into the chamber has been suitably dried to prevent excessive condensation (which takes absolutely ages to get rid of in the next evacuation).

And a few other things I can barely remember from my days operating a 'Do it yourself' BET rig for microsurface analysis of catalyst substrates.. pm if any of this makes sense?