• Oarasawa Formation

• Obama Formation
• Obara Tonalite
• Obata Formation
• Obatake Granodiorite
• Obiki Unit
• Oboke unit
• Obusuma Group

• Ochiai Conglomerate Formation
• Ochiai Formation

• Odakegawa Lavas
• Odokorogawa Formation

• Ofuji Lava
• Ofuna Formation
• Ofuna Mudstone Formation
• Ofunato Subbelt

• Oganomachi Formation
• Oganomachi Group
• Ogawa Formation
• Ogigakura Tuff
• Ogikubo Sandy Mudstone Member
• Ogi Quartz Diorite
• Ogo Pumice Flow Deposit

• Ohachidaira Caldera Eruption Deposit
• Ohachimori Amphibolites
• Ohashi Formation
• Ohatayama Lavas
• Ohatayama Pyroclastic Rocks
• Ohirayama Unit
• Ohno Formation
• Ohno Volcanic Rocks
• Ohyamashimo Formation
• Ohzone Formation

• Oi Formation
• Oishi Formation (岩手)
• Oishi Formation (御荷鉾ユニット)
• Oishigawa-futamata Lavas
• Oishigawa Lavas
• Oishigawa Pyroclastic Flow
• Oishigawa Volcanic Rocks
• Oishi Lava

• Ojo Formation

• Okamotsugawa Complex
• Okamura Formation
• Okanaro Group
• Okawaratoge Pyroclastics
• Okinazawa Basalt Member
• Okuda Formation
• Okuda Tuff Bed
• Oku Formation
• Okukurobe Granite
• Okumatashiro Granite
• Okushibetsu Plutonic Complex
• Okuwa Debris Avalanche Deposit
• Okuwahara Sandstone Member
• Okuyama Formation
• Okuzure Formation

• Old dune, Shonai sand dunes
• Older Atosanupuri Volcano
• Older Inbi stage intrusive rocks
• Older Kitadake Volcano
• Older Minamidake Volcano
• Older Ryoke Granitoids
• Older stage Hokusatsu Volcanic Rocks
• Older stage Nansatsu Volcanic Rocks
• Older Stratovolcano (Akagi Volcano)
• Oldest stage Nansatsu Volcanic Rocks

• Omae Unit
• Omagari carbonate islet
• Omagari Formation
• Omama Complex
• Omi Complex
• Omi Limestone
• Omi Metamorphic rocks
• Omine Acidic Tuff Bed
• Omine Pyroclastic Flow Deposit
• Omine Welded Tuff Bed
• Omma Formation
• Omma Sandstone Formation
• Omori Formation
• Omuro Lava

• Onagata Subunit
• Onbetsu Group
• Onigatsuji Mudstone and Sandstone Member
• Onishi Conglomerate Member
• Onma Formation
• Onma Sandstone Formation
• Onnagawa Formation
• Onzawa Facies

• Oppata Granite

• Oritate Conglomerate
• Oritate-toge Conglomerate
• Orito Formation
• Oro Formation
• Orojidani Formation

• Osaka Dacite
• Osaka Group
• Osakamoto Formation
• Osakatoge Complex
• Osakatoge Formation
• Osakihana Dacite
• Osawa Formation (Inai Group)
• Osawa Formation (Tanzawa Group)
• Osawaguchi Tuff Member
• Osawa Tuff Formation
• Oshidomari Lava
• Oshidomari-pon'yama Lava Dome
• Oshima Belt
• Oshima-Kojima volcano
• Oshimayama Subunit
• Oshimojo Formation
• Oshirasawa Granite
• Osoushinai Formation
• Osumi Granodiorite

• Otakidani Formation
• Otani Lavas
• Otanoshike Formation
• Otogawa Formation
• Otoge Volcanic Complex
• Otonashigawa Belt
• Otonashigawa Group
• Otori Formation
• Otori Unit
• Otsukamiya Granite

• Oyabe Formation
• Oyabe Mudstone Formation
• Oyakkotsu Lava Dome
• Oyamada Formation
• Oyama Formation, Hayama Group
• Oyama Formation, Tokai Group
• Oyama Subgroup
• Oyama Tuffaceous Sandstone Formation
• Oyashirazu Formation
• Oyashirazu Volcanic Rocks

• Ozenji Formation