- Oedilaaiffe by J. Thomas Crisler (48 features)
- Oedilaiffe by Duduko Echethesi (101 features)
- Oklane by David Johnson (63 features)
- Oksa language by k1234567890y (132 features)
- Okuna by Matt Pearson (122 features)
- Old Adpihi by eldin raigmore (95 features)
- Old Arassenian by Fanael (124 features)
- Old Balres by triskara (21 features)
- Old Drengan by Jack Lockwood (89 features)
- Old Greedian by Mardikhouran (130 features)
- Old Kaiese by GohtCheese (14 features)
- Old Lonmai luna by k1234567890y (96 features)
- Old Nevotak by k1234567890y (85 features)
- Old Northern by Dúrfin (0 features)
- Old Olgish by Michael M. Goessler (58 features)
- Old Pakhriban by DecadentMuffin (52 features)
- Old Palen by Paolo Barresi (46 features)
- Old Soskish by Michael M. Goessler (35 features)
- Old Sumrë by Marcas Ó Domhnaill (76 features)
- Old Velosti by Michael M. Goessler (37 features)
- Old Zarcosian by Matrix (126 features)
- Olgittic by Milyamd (48 features)
- Oligo by Brv (40 features)
- Oltic by Dillon Hartwig (0 features)
- Omayna by davush (122 features)
- Omino by Owen Gallagher (38 features)
- Omluuet by Creyeditor (70 features)
- Omniglot forum language by Members of omniglot forum (19 features)
- Omni-kan by Qwynegold (56 features)
- Omnizzabon by Becca (94 features)
- Onnasurin by Friederich Kugelschreiber (53 features)
- Oo by Xing (49 features)
- Oopian by Charles Koster, Debie nohr (0 features)
- oops by Joersk (0 features)
- Opus by Ariadne Smiith (33 features)
- Orenian by Bryony (38 features)
- Orin by skyta (18 features)
- Osklisk by Zoe Kirk-Robinson (10 features)
- Oter Tov Svik by Catherine B. Krause (4 features)
- Otrysontogantian by krysX (0 features)
- Ouxu by Jay Scott (61 features)
- Ox-yew by Jamin Johnson (49 features)
- Ozavakve by Solarius (91 features)