Common words:
* Open
* Over
* Out
* On
* Off
* Only
* Once
* Orange
* Ocean
* Other
* Old
* Often
* One
* Olive
* Onion
* October
* Order
* Oxygen
* Opportunity
* Opinion
Less common words:
* Obtuse
* Oafish
* Oblique
* Olfactory
* Omnipotent
* Opalescent
* Ostracize
* Ominous
* Orthography
* Oxymoron
Nouns:
* Owl
* Octopus
* Oak
* Organ
* Object
* Oasis
* Outfit
* Outcome
* Obstacle
Verbs:
* Observe
* Obtain
* Occupy
* Offer
* Operate
* Oppose
* Omit
* Overcome
* Overlook
Adjectives:
* Obvious
* Open
* Odd
* Old
* Orange
* Original
* Optimistic
* Outstanding
* Overjoyed
This is just a small selection of words, and there are many more words in English that begin with the letter O.