1. For the most part
   - principally, primarily, mainly, in the main, majorly

See also: chief

• onus
• onward
• onward motion
• onwards
• onya
• onycha
• Onychium
• Onychogalea
• onycholysis
• Onychomys
• Onychophora
• onychophoran
• onychosis
• onymous
• onyx
• onyx marble
• onyxis
• oocyte
• oodles
• oogenesis
• ooh
• oolite
• oology
• oolong
• Oom Paul Kruger