The Yukes are the only ones we've seen with Plot-Seeking Missiles®, capable of shooting down main story characters(Bartlett(M2) and Nagase(M14)). Don't know that the OADF can compete with that.
The Oseans couldn't break through or bypass the fortresses guarding the coastline(Lit Fuse) or the capital(Fortress) without extremely coordinated air support, and the Osean Air Force as a whole doesn't seem that great.
(Seriously, pilots who were trainees at the beginning of the war(Wardog) "had become the most experienced pilots in the entire war", according to the M6 cutscene. I don't believe the Yukes ever went on the offensive with either sub against the Osean mainland(i.e. any other airbases besides Sand Island), so either all the veteran OADF pilots had resigned and rejoined in the OMDF(and were thus stationed on the Buzzard and Vuture when they were sunk), or the OADF was apparently made up entirely of nuggets who were incapable of pulling back on their control sticks and engaging their afterburners to climb above 5000 feet when commanded, or of performing in-flight refueling.
Furthermore, ACZ showed us that the Yukes really didn't do anything during the Belkan War, except 'give' a few pilots to AWWNB. This explains why they could afford to put up such formidable coastal defenses and giant fortresses(and make giant submarines). The Gray Men were apparently in denial about being crushed by a handful of 'mere' mercenaries from Ustio, and must have deluded themselves into attributing Galm's successes to the Yukes.
If the Belkans started it all, as in AC5, the Gray Men would be a wild card, though. They'd have to step in to balance the scales in either case, to ensure the mutual destruction of both countries.
*"Plot-Seeking Missile"® is a not-really registered trademark of the Union of Timerlanian Republics, the Strangerealworld's leading producer of fiat-based weaponry. Void where prohibited, restrictions may apply.