O
- Cortinarius obtusus
- Cortinarius occidentalis
- Cortinarius odorifer
- Cortinarius olivaceopictus – edible, but deadly look-alikes (C. gentilis)
- Cortinarius olympianus
- Cortinarius orellanosus
- Cortinarius orellanus – (Europe, North America)
- Cortinarius orichalceus
- Cortinarius osmorphus
Read more about this topic: List Of Cortinarius Species