생일 축하해요, 오빠! or 오빠, 생일 축하합니다! either is fine.

It doesn't matter if you say 오빠 (oppa) in the beginning or at the end as it is in English.

축하해요 sounds less formal and friendlier than 축하합니다 or 축하드립니다 which you would use if you don't know him well or he is an age that you could even naturally call him 아저씨.

i think your wish will be "no war"  in this time