daikon/ʹdiek(ə)n, ʹdiekon/ noun

• once and for all/once for all
• once bitten, twice shy
• once in a blue moon
• once in a while
• once or twice
• once-over
• oncer
• once upon a time
• onchocerciasis
• on cloud nine
• oncogene
• oncogenic
• oncogenous
• oncology
• oncoming
• on commission
• on condition that
• oncost
• on credit
• on cue
• OND
• on demand
• on deposit
• on display
• on dit