Badve:
There are many precautions that I could easily list out for you involving the transfer of 350 barg Oxygen. However, the best and most authoritative way to learn about the hazards and techniques involved is to buy & read the CGA (Compressed Gas Association) book on compressed gases titled: "Handbook of Compressed Gases". In it, you'll quickly be oriented with the criteria and the subject matter involved in applying and transferring Oxygen and other gases.
As an example of what you should be looking out for is the inclusion of oil or grease inside piping meant for Oxygen use. I highly recommend you insist on a 100% assurance that all piping is 100% oil/grease or hydrocarbon-free. This includes checking that any water used in a hydrostatic test on the pipe is totally oil and grease-free. Additionally, I would run an oil and grease solvent through the piping to ensure that it is, in fact, free of any hydrocarbon substance prior to being filled with Oxygen. (of course, you have to purge and evaporate the residual solvent - more expense and labor, typical of Oxygen installations). The big concern in an Oxygen application is not the high pressures used or the gas itself; it is the potential, dangerous mixture of pure Oxygen with a readily oxidizable substance, such as a hydrocarbon. This is never to be allowed or credibly achievable. Mechanical engineers will recognize the product of such a mixture as an explosion.
Also, bear in mind that the B31.3 Process Piping Code is a mechanical code; this has nothing to do with the ultimate hazard and danger when dealing specifically with Oxygen: it will oxidize efficiently and instantly if given the right opportunity - in other words, it will detonate any hydrocarbon it comes in contact with under the right conditions. Your application is very process sensitive and you should consider the process needs and precautions rather than just applying strictly mechanical codes in his instance. There are existing standards for Oxygen service within some organizations - especially producers like Air Products, Praxair, Liquid Air, etc. These people can give you all the correct and safe guidelines to put your Oxygen system into service under the best and most experienced guidelines and standards - besides those of B31.3. You may, in fact, be purchasing your Oxygen from such suppliers and it would be a smart move to get their expert advice on how to test, clean, and ensure a safe Oxygen piping system.