Merriam-Webster's Collegiate Dictionary:
oct-
octa-
octagon
octagonal
octagonally
octahedral
octahedrally
octahedron
octal
octameter
octane
octane number
octane rating
octant
octapeptide
octave
octavius
octavo
octet
octillion
octo-
october
octodecillion
octogenarian
octoploid
octopod
octopus
octoroon
octosyllabic
octosyllable
octothorp
octothorpe
ocular
ocularist
oculist
oculomotor
oculomotor nerve
oculus
od
odalisque
odd
odd fellow
odd lot
odd man out
odd permutation
odd trick
odd-pinnate
odd-pinnately
oddball
oddity
oddly
oddment
oddness
odds
odds and ends
odds-on
oddsmaker
ode
odense
oder
oderint dum metuant
odesa
odessa
odets
odeum
odi et amo
odic
odiferous
odin
odious
odiously
odiousness
odist
odium
odoacer
odometer
odonate
odont-
odonto-
odontoblast
odontoblastic
odontoglossum
odontoid process
odor
odorant
odored
odoriferous
odoriferously
odoriferousness
odorize
odorless
odorous
odorously
odorousness
odour
odovacar
odovakar
