If OLA had the weapons, funds, etc that the TDF led Tigrayans have, it'd be over quicker. However, even with that being said, the land OLA administers is bigger than Tigray. Tigrayans were preparing for years for this. The amount of weapons they've stashed is crazy man. Not to mention that their 250,000 strong police and militias are all United and joined TDF after initially scattering when Abiy and his allies first struck Tigray.
Oromia Police is split between those fighting for Abiy against OLA, for the sake of their paychecks, and the other part which is refusing to fight, some of which are joining OLA. This is why Eritrean troops and Amhara troops also are in parts of Oromia. Especially in the west where the OLA has a strong presence.
Our local government was created by TPLF remember? To counteract the OLA? So of course they wouldn't be as loyal to their people as the TPLF is to Tigrayans.