This list contains all the adverbs used in the English language that start with O. It includes negative and positive adverbs with O to describe a person.
| Oafishly | Oathfully | Obdurately |
| Obediently | Obeisantly | Obeyingly |
| Obiter | Obituarily | Objectively |
| Obligatorily | Obligedly | Obligingly |
| Obliquely | Obliviously | Oblongly |
| Obnoxiously | Obscenely | Obscurely |
| Obsequiously | Observably | Observantly |
| Observingly | Obsessively | Obsoletely |
| Obstinately | Obstreperously | Obtrusively |
| Obtusely | Obversely | Obviously |
| Occasionally | Occultly | Occupiedly |
| Ocularly | Oddly | Odiously |
| Odorously | Off | Off-balancingly |
| Offendedly | Offensively | Offhandedly |
| Officially | Officiously | Off-key |
| Off-tune | Oft | Often |
| Oftener | Oftentimes | Ofter |
| Ofttimes | Oilily | Ominously |
| On | Ontheotherhand | Once |
| One-handedly | Onerously | Ones |
| Onethe | Online | Only |
| Onto | Onward | Onwardly |
| Onwards | Opaquely | Openhandedly |
| Openly | Open-mindedly | Open-mouthedly |
| Operosely | Opinionatedly | Opportunely |
| Opportunistically | Opposedly | Opposite |
| Oppositely | Oppressively | Optatively |
| Optically | Optimally | Optimistically |
| Optionally | Opulently | Oracularly |
| Orad | Orally | Orderingly |
| Ordinarily | Ordinately | Organizationally |
| Organizedly | Organizingly | Originally |
| Ornamentally | Ornately | Ornerily |
| Orthodoxly | Oscitantly | Ostensibly |
| Ostentatiously | Other | Othergates |
| Others | Otherways | Otherwhere |
| Otherwhile | Otherwise | Out |
| Outcept | Outdoors | Outerly |
| Outlandishly | Outragedly | Outrageously |
| Outrightly | Outside | Outspokenly |
| Outward | Outwardly | Outwards |
| Ovally | Over | Overall |
| Overbearingly | Overboard | Overconfidently |
| Over-eagerly | Over-enthusiastically | Overhead |
| Overhighly | Over-joyedly | Overleaf |
| Overly | Overmore | Overmuch |
| Overprotectively | Overseas | Overside |
| Oversoon | Overtime | Overtly |
| Overwhelmedly | Overwhelmingly | Over-zealously |
| Owlishly | Oxymoronically |
