Two problems:
(1) The RPGs (and most other explosives) in OFP are of the non-shrapnel variety, the airbursts you're talking about kill infantry thanks to the flechettes/shrapnel they rain down on their targets, and not because of the actual explosive shockwave.
(2) In OFP M203 grenades, handgrenades, bombs, missiles, etc all explode only after coming in contact with some object (other than air).
So if you want an RPG round to airburst you'd have to script some object/game logic to appear in the path of the grenade so that it'll blow.
However, I know that snYpir's "SP/MP Support Pack" (I think) does simulate an airburst with actual shrapnel.
So you might want to ... ahem! "hunt him down" and ... err... "extract" his secrets from him. ;D