OP has the advanced weapons. Maulers, MIRVs, X-phasers, Heavy Heavies. The Orions are more detailed. Other things are that there is more ship spots available for OP. I think EAW ship upper limit is 64 per class, while OP is 128 per class. (something along those lines anyway, OP has 2x as many ship slots available). What that means is there's more room for variants and conjectural ships in the shiplist for OP, EAW the list would need to be pared down further because there just isn't enough room for them. There also a few other minor differences, given that OP is the later of the two and was improved further than EAW. The two games play (mostly) the same, and use the same technology. OP was sold as a stand alone expansion, that didn't require EAW to play, but the game is by and large the same, with just some minor differences, and of course, the extra weapons.