O nomenclature (from the IAU and from unofficial sources)

Italics indicate farside features.

-Menzel, 1971- O'Day
-Wilkins- O'Kell (Atlas E)
-Wilkins- O'Neill (+730/ +260)
O'Neill's Bridge (unofficial name)
Oberth
-Firsoff?- Object A (Mare Ingenii)
Oblivionis, Lacus
-Menzel, 1971- Obruchev
-Van Langren- Ocariz (IAU: Julius Caesar)
-Hewelcke- Occidentalior, Insula (location unknown)
Odii, Lacus
-Riccioli- Oenopides
-Maedler- Oersted
-NASA- Offset (Apollo-15 site)
-Menzel, 1971- Ohm
-Maedler- Oken
-Maedler- Olbers
-rille seeker- Olbers, Rimae (on the floor of Olbers)
-Franz- Olblatt (Hansen B) (Wilkins's Lower)
-Menzel, 1971- Olcott
-Caes- Olcott, Catena
-NASA- Old Deep (Apollo-14 site)
-NASA- Old Nameless (Apollo-14 site)
-NASA- Old North (Apollo-14 site)
Olivier
-Birt- Olivium, Promontorium (discontinued name?)
-Hewelcke- Olympus, Mons (IAU: Hipparchus)
-Menzel, 1971- Omar Khayyam (in Poczobutt)
Oncoming hills (Luna 21 / Lunokhod 2 site) (in Le Monnier)
-RP 1097- Onizuka (crewmember STS-51L)
-Schmidt- Opelt
Opelt, Rimae
-Hewelcke- Ophiusa, Insula (IAU: Montes Teneriffe Delta, etc.)
Oppel, Dorsum
-Menzel, 1971- Oppenheimer
-rille seeker- Oppenheimer, Rimae (on the floor of Oppenheimer)
-Krieger- Oppolzer
Oppolzer A (very near the point at 0°North/ 0°East)
Oppolzer, Rima
-Van Langren- Opticus, Sinus (IAU: Sinus Amoris)
-Menzel, 1971- Oresme
-Maedler- Oriani* (discontinued name)
-Hewelcke- Orient, Paludes (NW edge of Oceanus Procellarum)
-Franz- Orientale, Mare (Basin)
Orientale Dark Mantle Ring Deposit (aka Mare Pacificus)
Orientale Secondary chains
-Hewelcke- Orientalior, Insula (location unknown)
-Gilbert- Orientalis, Sinus (IAU: probably north of Mare Humorum)
-Hewelcke- Orientalis, Sinus (western end of Mare Frigoris)
-Riccioli- Origanus (Darney Tau and Chi)
Oris (see Brisbane, Brisbane Z) (and description below list)
-Menzel, 1971- Orlov
-Riccioli- Orontius
-Wilkins- Orus (Petavius B)
-NASA- Orville (Orville Wright) (Apollo-15 site)
-NASA- Os (Apollo-15 site)
Osama (craterlet near the D-shaped Ina caldera)
-Sternberg- O.Schmidt (see Schmidt)
Osiris (craterlet near Mons Argaeus)
Osman (craterlet in Catena Davy)
-Van Langren- Ossolinski (IAU: Sirsalis A) (Wilkins's Bertaud)
-Menzel, 1971- Ostwald
-Riccioli- Osymandies (IAU: Mercurius)
-NASA- Outhouse Rock (Apollo-16 site)
Owen, Dorsum (near Aratus CA, aka "Lorca")
-?- Owl, The (Aristarchus/Herodotus/Vallis Schroteri)
-Van Langren- Oxensterni (IAU: Snellius)