I did a route like this last week (XIY-ICN on KE, 4 hr layover then ICN-LAX on OZ. The ticket including both segments was sold by OZ.

My luggage was routed all the way, no need to recheck it at ICN.

You don't need a visa as you won't be leaving the airport. You do need to go through a security line on arriving at ICN.

Note that as from some date in January, KE is moving to the new Terminal 2 at ICN while OZ stays put in T1. Don't know if this would make a difference.