O	əʊ	same as owe
obvious	ˈɒbviːəs
ocean	ˈəʊʃən	cf. version
oceanic	əʊsiːˈænɪk	cf. association
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	ˈɒnɡəʊ ɪŋ	jocularly ˈɒŋˌɔɪŋ
oral	ˈɔːrəl	same as aural
orange	ˈɒrɪndʒ	doesn't rhyme with silver
ordinary	ˈɔːdɪnəriː	or ˈɔːdnəriː
oregano	ˌɒrɪˈɡɑːnəʊ	rhymes with Arno
ostrich	ˈɒstrɪtʃ	cf. spinach
our	ɑː	rarely ˈɑʊ ə