What does your acrostic spell out? What is the poem about?
Ostentatious, officious, obsequious, obscene, obnoxious, ominous
Ontario, Oregon, ocean - geographical locations could be used out of context, as a description- for example, the band Soundgarden uses a lyric "Looking California/Feeling Minnesotta" I think that's it-
on, or, out, off
oust, observe, ogle
onslaught, octopus, oar...
rhymezone's a good idea.