This edited post is specifically for the OFF mission vs Khaak ships; general applications still apply however..
Osaka: big, ugly and slow. The big is no issue as Kha'ak Kyons hit targets of all sizes; ugliness does not unfortunately cause seizures on opponents. Being slow is the worst feature. As long as you can outrun/outmaneuver your enemies, you control whom you take punishment from. Osaka is potentially slower than any Kha'ak.
PSG is an area effect weapon not to be used near friendly objects. Good for Kha'ak sector clearouts, however, PSG has relatively short range. FLAK is very effective against fighters, but does not harm bystanders like PSG.
PPC (Photon Pulse Cannons) outranges all other guns. As long as you can stay out of range of your enemy and still damage them, it does not matter how nasty the guns they have and how weak your shield is. Trust me, I've met Kha'ak, Xenon and Terran ships and they all shared the same trait: they died. That said, the Kha'ak are numerous and the small ones will get close to you. Shielding is required. Osaka has 12GJ, but 10GJ of say (Argon) OTAS Boreas is almost as good. Boreas is faster than Osaka and can mount both PPC and FLAK.
For the record, in final FF missions I did fly Split Python that has measly 6GJ shielding and no effective anti-fighter weaponry.
Split Tiger and Panther are M7 to consider. The Tiger should mount IBL (Incendiary Bomb Launcher) on front and FLAK elsewhere, while the Panther has a bay for a serious (expendable) fighter wing. There is a catch in IBL: only the Yaki and Pirates manufacture them. You should either gain reputation to trade with them, or board ships that carry IBL's.
Your combat rank increases only by personal kills. The actions of your fleet do not credit your rank. A skilled admiral can be Harmless, yet leave devastation at her wake.
The last is specifically relevant for the OFF final mission: the higher your combat rank when you accept that mission, the more powerful the ships will be to oppose you (& your fleet, should you choose to take one into that sector)!