- Oatso Simple
- Large SW-trending tube at start of Chile in Steinschlagschacht (1999)
- Oldham Pitch
- First pitch of Flat Battery, before PP3 (1990)
- Olympus
- Area of horizontal passages off west from Tower Blocks, also spelt Olypus
due to initial writo (1988-1989)
- Oral Series
- Rift pitch series down from Dreaming of Limo (1995)
- Orchestral Pit
- Lower
chamber below The Theatre in Steinschlagschacht (1997)
- Orient Express
- 48m Pitch below Algeria in France (1992)
- The Oubliette
- Short pitch below Butcher in Left Hand Route (1988)
- Outright Rejection
- 16m fourth pitch of Interview Blues (1996)
- The Overflow
- Waterfall (good carbide filler) marking change from Triassic Park I to
Triassic Park II (1995), and the short pitch series below this (1998)
- The Overshoot
- Large hading passage trending SW beyond Oatso Simple in Chile (1999)
- Over the Rainbow
- Complex widening of Chunnel at traverse over Rainbow Pitch in Right Hand
Route (1988)