| Oili | | OY-lee | Finnish | |
| Oiva | | OY-vah | Finnish | |
| Olavi | | O-lah-vee (Finnish) | Finnish, Estonian | |
| Oliver | | AHL-ə-vər (English), AW-lee-ver (German), O-lee-ver (Finnish) | English, German, Swedish, Norwegian, Danish, Finnish, Estonian, Serbian, Croatian, Macedonian, Czech | |
| Olivia | | ə-LIV-ee-ə (English), o-LEE-vyah (Italian, Spanish), o-LEE-vee-ah (German), O-lee-vee-ah (Finnish) | English, Italian, Spanish, German, Finnish, Swedish, Norwegian, Danish | |
| Olli | | OL-lee | Finnish | |
| Onni | | ON-nee | Finnish | |
| Oona | | OO-na (Irish), O:-nah (Finnish) | Irish, Finnish | |
| Orvokki | | OR-vok-kee | Finnish | |
| Oskari | | OS-kah-ree | Finnish | |
| Osku | | OS-koo | Finnish | |
| Otso | | OT-so | Finnish | |
| Otto | | AW-to (German), AH-to (English), OT-to (Finnish) | German, Swedish, Norwegian, Danish, Dutch, English, Finnish, Ancient Germanic | |