... just how clean does the air have to be and stay safe?...
I will not offer any advice. But I will give you my real-world experience!
I use an Alkin W-31 oil-based compressor with a small integral air filter.
Air anlysis from the integral air filter meets all oxygen-compatable air (OCA) standards except dew point. Then, I run the air through a 24-inch LF filter and it comes out so clean and dry that it greatly exceeds all OCA standards.
I routinely fill three HC-4500 storage cylinders with EAN32 to 4,000 PSI by continuous blending.
I also do partial-pressure mixing and top off with continuous-blend EAN40 to make mixes in excess of 40%.
I completely disassemble everything down to machined parts (except the compressor) every year and perform a thorough oxygen cleaning.
I never leave the compressor unattended when it is running. I have a very large ABC fire extinuigsher at my escape route.
Can you be too safe? How much is your house worth? What would your house look like with a big fire fueled by high-pressure oxygen? We've all got to determine how much risk we are going to take.
Clearly, my risk is running EAN40 though an oil-based compressor at 4,000 PSI. But I've never had any problems in 3 years of operation.