EPR (look at air-oil.com for a source) O-rings work just fine.
The only one to be careful with is the small piston HP O-ring. I've had no trouble with 70 duro EPR in there on the Mk10s up to 3500 psi or so (the max I actually use) but SP is currently using a harder compound - probably 80 or 90 duro, and rumor has it its a Urethane compound - and those are hard to get.
The 70 duro EPR O-rings in a Mk20 or 25 (with the plastic bushing system) WILL LEAK over 2500-3000 psi.
You want the "E-70" ones.
EPR is reasonably-compatable with high FO2s and doesn't suffer from the poor abrasion resistance of Viton. Its also a hell of a lot cheaper than Viton is.
The ONLY unfortunate thing with them is that they don't stock 90 duro EPRs. That sucks, because if they did they'd have everything in the O-ring department for scuba gear.
They can GET 90 duro EPR, but the minimum orders are at the several-hundred-piece level (per size!) for them.
I've bought and used a lot of their EPR O-rings and have been happy with them - they're quite reasonably priced and of excellent quality.
I've considered ordering up their minimum quantity of the -10s in 90 duro EPR, as that would likely last me for many years - we're talking like 600 pieces or so. -10s are used in a number of different places, with the most important (for this purpose anywhere) being the HP Piston ring.