1 hr 20 min should be just fine. Usually UA arrives in the higher A gates. Not sure where OZ operates from, but my guess would be S. There’s a quick airside tram. Really it shouldn’t take anymore than 20-30 minutes max to get from the furthest gates to anywhere in SEA. And that’s on the high end. Pre-Covid the concern at SEA was gate availability, but lately that hasn’t been an issue. The only concern I would have with this itinerary is if for some reason you had to claim luggage and recheck in with OZ if things weren’t checked all the way through. Security lines at SEA can back up frequently if they have some closed. But it doesn’t sound like this is the case here. Have a good trip!