We don't have synonyms for oncer yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are once, oner and boncer.
1944: Could he but once see Nature as / In truth she is for ever, / What oncer would not fall in love? -- W. H. Auden, The Sea and the Mirror (Part 1. Prospero to Ariel) "A commentary on Shakespeare's The Tempest"
1944: Could he but once see Nature as / In truth she is for ever, / What oncer would not fall in love? -- W. H. Auden, The Sea and the Mirror (Part 1. Prospero to Ariel) "A commentary on Shakespeare's The Tempest"