- opsin
- obsolescence
- obsolescent
- optocoel
- ora
- orbit
- ordinal
- organelle
- organicism
- organoid
- organophyly
- organule
- orthocephaly
- osculant
- oscule
- osmeterium
- osmosis
- osmotic
- osteogenesis
- organ
- organogenesis
- organogenic
- organology
- organotrophic
- orthoblastic
- orthodromic
- orthogamous
- orthogamy
- orthoploidy
- orthoradial
- orthospermous
- outbred
- ovate
- osculate
- osmose
- osteogenetic
- ostosis
- ovocyst
- ovular
- ovule
- oxychromatin
- oxyhemoglobin
- paedogenesis
- palaceous
- palmatilobate
- ovicell
- ovisac
- oxidase
- oxylium
- pachynesis
- palingenesis
- panmixia
- panspermatism
- paedogamic
- paedogamy
- pairing
- paliform
- pallial
- palmatipartite
- papillary
- papula
- papulose
- parabiosis
- paracasein
- paracyst
- paraderm
- parafrontals
- parakeratosis
- paranucleus
- pangen
- panphotometric
- pantostomatic
- papilla
- papillae
- papulae
- papular
- papulous
- parablast
- parabranchia
- parachromatine
- paracme
- parademe
- paraplasm
- paraselectivity
- parasite
- parasitize
- parasympathetic
- parasynapsis
- paravariation
- paravascular
- paraxial
- parenchymal
- parenchymatous
- paries
- parietes
- paroecious
- parthenogamy
- parthenogenesis
- parthenogenetic
- patroclinous