neuron, cord, nerve: neural, neur-algia (algos, pain), neuritis.
nomos, a law: eco-nomy (oikos), astro-nomy, anti-nomian.
Ōdē, a song: ode, hymnody, parody.
oikos, oikia, house; oikēsis, inhabiting, dwelling: eco-nomy (Lat. oeconomia), di-ocese (= di-oikesis), parish (paroikia, Fr. par-oisse).
onoma (onuma), name: syn-onym.
op-, see; optika, pertaining to sight; oph-thalmos, eye: op-tics, optician, syn-opsis, synoptic, my-opia.
organon, an instrument: organic.
orthos, right, straight: ortho-doxy, ortho-graphy.
osteon, a bone: oste-itis, osteo-pathy.
oxus, sharp: oxy-gen, oxy-tone, par-oxysm.