AIR FORTRESS (O is the letter)
 
First Quest:
Stage 2: I8TA
Stage 3: KA9I
Stage 4: 6KAI
Stage 5: 8NYU
Stage 6: 2APP
Stage 7: OS85
Stage 8: SUGA

Second Quest:
Stage 2: 11NA
Stage 3: 5AJI
Stage 4: TOBI
Stage 5: NDA4
Stage 6: MA1K
Stage 7: DOMO
Stage 8: 7NDE