1. Extremely dark
   "it was pitch-dark in the cellar"
   - black, pitch-black

See also: dark

• Onoclea sensibilis
• Onoclea struthiopteris
• onomancer
• onomancy
• onomastic
• onomasticon
• onomastics
• onomatomania
• onomatopoeia
• onomatopoeic
• onomatopoeical
• onomatopoetic
• Onondaga
• Ononis
• Ononis repens
• Ononis spinosa
• Onopordon
• Onopordon acanthium
• Onopordum
• Onopordum acanthium
• Onosmodium
• onrush
• Onsager
• onset
• onshore