O

O əʊ
of ɒv reduces to əv, ə
often ˈɒfən
ogle ˈəʊɡəl like mogul, not goggle
ominous ˈɒmɪnəs
omniscience ɒmˈnɪsiːəns
one wən same as won
onerous ˈəʊnərəs
ongoing ˈɒnɡəʊɪŋ jocularly ˈɒŋˌɔɪŋ
oral ˈɔːrəl same as aural
orange ˈɒrɪndʒ doesn't rhyme with silver
ordinary ˈɔːdɪnəriː or ˈɔːdnəriː
oregano ˌɒrɪˈɡɑːnəʊ rhymes with Arno