Pholas /fōˈləs/

1. The piddock genus of rock-boring molluscs
2. (without cap; pl pholades /fōˈlə-dēz/) a mollusc of this genus

• once bitten, twice shy
• once-errand
• once for all
• once-for-all
• once in a blue moon
• once in a way
• once in a while
• once more
• once or twice
• once-over
• once-overs
• oncer
• oncers
• onces
• once upon a time
• onchocerciases
• onchocerciasis
• oncidium
• oncidiums
• on cloud nine
• onco-
• oncogen
• oncogene
• oncogenes
• oncogeneses