Observation, 1924-1942
- O-1 Falcon - Curtiss
- O-2 - Douglas
- XO-3 Mohawk - Dayton-Wright
- XO-4 - Martin
- O-5 - Douglas
- O-6 - Thomas-Morse
- O-7 - Douglas
- O-8 - Douglas
- O-9 - Douglas
- XO-10 - Loening
- O-11 Falcon - Curtiss
- O-12 Falcon - Curtiss
- O-13 Falcon - Curtiss
- XO-14 - Douglas
- XO-15 - Keystone
- XO-16 Falcon - Curtiss
- XO-17 Courier - Consolidated
- XO-18 Falcon - Curtiss
- O-19 - Thomas-Morse
- YO-20 - Thomas-Morse
- XO-21 - Thomas-Morse
- O-22 - Douglas
- YO-23 - Thomas-Morse
- O-24 - Curtiss
- O-25 - Douglas
- Y1O-26 - Curtiss
- Y1O-27 - Fokker
- O-28 Corsair - Vought
- O-29 - Douglas
- O-30 - Curtiss
- O-31 - Douglas
- O-32 - Douglas
- Y1O-33 - Thomas-Morse
- O-34 - Douglas
- O-35 - Douglas
- XO-36 - Douglas
- O-37 - Keystone
- O-38 - Douglas
- O-39 Falcon - Curtiss
- O-40 Raven - Curtiss
- Y1O-41 - Thomas-Morse
- O-42 - Douglas
- Y1O-43 - Douglas
- XO-44 - Douglas
- O-45 - Martin
- O-46 - Douglas
- O-47 - North American
- XO-48 - Douglas
- O-49 Vigilant - Stinson (redesignated L-1 modernistic 1942)
- YO-50 - Bellanca
- YO-51 Dragonfly - Ryan
- O-52 Owl - Curtiss
- O-53 Havoc - Douglas
- YO-54 - Stinson
- YO-55 - ERCO
- O-56 Ventura - Lockheed
- O-57 Grasshopper - Taylorcraft (redesignated L-2 now 1942)
- O-58 Grasshopper - Aeronca (redesignated L-3 modern 1942)
- O-59 Grasshopper - Piper (redesignated L-4 mod 1942)
- O-60 - Kellett
- YO-61 - Pitcairn
- O-62 Sentinel - Stinson (redesignated L-5 modernized 1942)
- XO-63 Grasshopper - Interstate (redesignated XL-6 now 1942)
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