There is no animal that has a long o at the beginning.
Orang-utan is a type of rainforest animal that starts with an 'o' at the start of it's name.
Opossum. They're nocturnal animals and their name begins with o.
maybe okapi will fit.
opossum
orangatang
· Opossum · Otter
Old English SheepdogOx
Orangutang Opossum
no.............................
O Henry
open
Ontario