observably
To a detectable degree, sufficient to be observed.
In an observable manner.
Indicates that the proposition can be verified by observation.

observantly
In an observant manner.

observationally
In an observational way.

observedly
In a way that is observed.

observingly
Attentively, observantly.

obsessedly
With an obsession.

obsessingly
With an obsession; obsessively.

obsessionally
In an obsessional manner.

obsessively
In an obsessive manner.
To an obsessive degree.

obsolescently
In an obsolescent manner.

obsoletely
When out of date or no longer in use.
Imperfectly or indistinctly developed.

obstetrically
In terms of obstetrics

obstinately
In an obstinate manner.

obstreperously
In an obstreperous manner.

obstructedly
While being obstructed.

obstructingly
So as to obstruct.

obstructionally
As, or by way of, an obstruction; obstructively.

obstructively
In an obstructive manner.

obtrusively
In an obtrusive manner.

obtusely
In an obtuse manner.

obv
Abbreviation of obviously.

obversely
In an obverse manner.
correspondingly

obviatively
In an obviative manner.

obviously
In an obvious or clearly apparent manner.
Used as a filler word, or to introduce information even when not obvious.

obvs
Clipping of obviously.

obvz
Alternative form of obvs (“obviously”).

occasionably
occasionally

occasionally
On occasion: at relatively infrequent intervals, from time to time, sometimes.
On an occasion, accidentally, by chance.
On the occasion of something else happening; incidentally, by the way.

occasionly
Misspelling of occasionally.

occassionally
Misspelling of occasionally.

occidentally
In an occidental manner.

occipitally
In relation to the occiput or the occipital bone.

occipitofrontally
In an occipitofrontal manner.

occipitolaterally
In an occipitolateral manner.

occlusally
Within, or associated with an occlusion

occlusively
In an occlusive manner.

occlusocervically
In the occlusocervical dimension or orientation.

occultically
In an occultic manner.

occultly
In an occult manner.

occupationally
as part of an occupation

occurrentially
In terms of occurrence.

occurrently
In an occurrent manner.

oceanically
In an oceanic manner; in a way that suggests or involves the ocean.

oceanographically
In terms of oceanography.

oceanologically
In terms of oceanology.

oceanwise
Towards or along an ocean.

ochlocratically
In an ochlocratic way.

octagonally
In an octagonal manner.

octahedrally
In an octahedral manner

octangularly
In an octangular manner.

octennially
Every eight years.

octonionically
In terms of octonions.

octuply
In an eightfold manner

ocularly
By means of the eye; visually.
Into or towards the eye.

oculonasally
In an oculonasal manner

oddly
In a peculiar manner; strangely; unusually.
In a manner measured by an odd number.

oddswise
With respect to odds.

odically
In terms of the hypothetical force called od.

odiferously
In an odiferous manner.

odiously
In an odious manner.

odometrically
By means of, or in terms of, odometry.

odontologically
In terms of, or by means of, odontology.

odoriferously
In an odoriferous manner.

odorlessly
Without an odor.

odorously
In an odorous manner.

odourlessly
Without an odour.

odourously
Misspelling of odorously.

oedipally
In a manner related to an Oedipus complex. Adverb form of oedipal.

oenologically
In an oenological manner; with regard to the study of wine.

oestrogenically
Alternative form of estrogenically.

offen
Pronunciation spelling of often.

offencelessly
Alternative form of offenselessly.

offencively
Alternative form of offensively.

offendedly
In an offended manner.

offenselessly
In an offenseless manner.

offensively
In an offensive manner.

offhandedly
in an offhand manner

offically
Misspelling of officially.

officeward
Toward an office.

officially
In an official manner; according to official rules or regulations.
Thoroughly, completely.

officinally
In officinal terms.

officiously
in an officious manner

offishly
In an offish manner.

offkey
in an off-key manner

offly
awfully

offputtingly
In an offputting manner.

offseason
Alternative form of off-season.

offseasons
plural of offseason

oft
often; frequently; not rarely

oftener
comparative form of often: more often

oftenest
superlative form of often: most often

oftenly
often

oftens
Synonym of often.

oftentide
Often; frequently.

oftentimes
Frequently; often.

oftenwhiles
often; frequently

ofter
comparative form of oft: more oft; more often

oftest
Most often (superlative form of oft: most oft)

oftimes
(elsewhere, archaic) Alternative form of ofttimes.

ofttime
often

ofttimes
(elsewhere, archaic) Often, frequently.

oftwhiles
Often; oftentimes.

ogreishly
Like an ogre.

ogrishly
In an ogrish manner.

ohmically
By means of ohmic heating, the process by which the passage of an electric current through a conductor releases heat.

oilily
Flatteringly, obsequiously.
In an oily way; with the appearance or consistency of oil.

okayly
In an okay manner.

oldly
In an old manner.

oleaginously
In an oleaginous manner.

oleographically
By means of, or in the style of, oleography.

oleophobically
In an oleophobic manner

olfactometrically
By means of olfactometry.

olfactorily
In an olfactory sense.

oligarchically
In an oligarchical way.

oligoclonally
In an oligoclonal manner

oligogenically
In an oligogenic manner

oligopolistically
In an oligopolistic manner.

oligotrophically
In an oligotrophic manner

omentally
In an omental manner

ominously
In an ominous manner; with sinister foreboding.

omissively
In an omissive manner; by omission.

omnidimensionally
In all dimensions.

omnidirectionally
In all directions.

omnifariously
In an omnifarious manner.

omnilaterally
In an omnilateral fashion.

omniphobically
In an omniphobic manner

omnipotently
In an omnipotent manner.

omnipresently
In an omnipresent manner.

omnisciently
In an omniscient manner; with omniscience.

omnisexually
In an omnisexual manner.

omnispatially
Throughout space; everywhere in space.

omnitemporally
In an omnitemporal manner

omnivorously
In an omnivorous manner.

onanistically
In an onanistic manner.

once
One and only one time.
Formerly; during some period in the past.
At any time; ever.
One day, someday.
Multiplied by one: indicating that a number is multiplied by one.

onced
Alternative form of oncet (“once”).

onces
plural of once

oncet
Once.

oncogenetically
By means of oncogenetics

oncogenically
In an oncogenic manner

oncologically
With regard to oncology.

onct
Alternative form of oncet.

onehandedly
Alternative form of one-handedly.

oneirically
In an oneiric fashion.

oneirocritically
In terms of dream interpretation.

onerously
In an onerous manner.

onest
Obsolete form of oncet.

ongoingly
In an ongoing fashion.

onomasiologically
In an onomasiological manner or context.

onomastically
In an onomastic way.

onomatologically
In terms of or by means of onomatology.

onomatopoeically
In an onomatopoeic manner.

onomatopoetically
In an onomatopoetic manner.

onomatopoietically
Alternative form of onomatopoetically.

onomotopoetically
By means of onomatopoeia.

onst
once

ontically
In an ontic sense.

ontoepistemologically
In terms of ontoepistemology.

ontogenetically
Synonym of ontogenically.

ontogenically
With regard to ontogeny

ontologically
In an ontological manner.
Inherently.

ontotheologically
In terms of ontotheology.

onwardly
onward

onwards
Onward.

onymously
In an onymous manner; having a name.

oogamously
In an oogamous manner.

oologically
From the perspective of scientific study of eggs or of a collector of eggs.

oozily
In an oozy manner.

oozingly
In an oozing manner.

opalescently
In an opalescent manner.

opaquely
In an opaque manner.

openhandedly
Alternative form of open-handedly.

openheartedly
Alternative form of open-heartedly.

openly
In an open manner; visibly.

openmindedly
Alternative form of open-mindedly.

openmouthedly
With the mouth open, especially in astonishment.