Fish
16 species| Photo | English | සිංහල | தமிழ் | Scientific name | IUCN | Endemic |
|---|---|---|---|---|---|---|
| Ocellated Damselfish | — | — | Neoglyphidodon bonang | LC | — | |
| Olive Barb | — | — | Systomus sarana | LC | — | |
| Ompok argestes | — | ஓம்போக் அர்ஜெசுடெசு | Ompok argestes | VU | Endemic | |
| Ompok ceylonensis | — | — | Ompok ceylonensis | LC | — | |
| Onespot Snapper | — | — | Lutjanus monostigma | LC | — | |
| Opisthopterus tardoore | — | — | Opisthopterus tardoore | LC | — | |
| Orange Chromide | — | — | Pseudetroplus maculatus | LC | — | |
| Orange-Socket Surgeonfish | — | — | Acanthurus auranticavus | LC | — | |
| Orangelined Cardinalfish | — | — | Taeniamia fucata | LC | — | |
| Orangestripe Triggerfish | — | — | Balistapus undulatus | LC | — | |
| Orbicular Cardinalfish | — | — | Sphaeramia orbicularis | LC | — | |
| Oriental Sole | — | — | Brachirus orientalis | LC | — | |
| Oriental Sweetlips | — | — | Plectorhinchus vittatus | LC | — | |
| Ornate Ponyfish | — | — | Nuchequula gerreoides | LC | — | |
| Ornate Sandgoby | — | — | Istigobius ornatus | LC | — | |
| Ovoid Ponyfish | — | — | Gazza dentex | LC | — |
