All families

elaia, = the olive tree
olea, = an olive;
oleum, = oil, olive oil.
(LS, ld)

For Johan Henrik Olin (1769-1824).**
(Ch)

onagros, = a wild ass; humorously ‘a she-ass’.
(a former name for the genus Œnothera)
onagos, = an ass-driver.
Its etymology is uncertain but is believed to be derived from theGreek words ονος θηρας (onos theras), meaning "donkey catcher," or οινος θηρας (oinos theras), meaning "wine seeker." In addition,oenothera means "a plant whose juices may cause sleep" in Latin.
(LS, Ox, ld, I6)

ophis, = a serpent, snake.
glossa, = the tongue.
(LS)

orchis, = a testicle, the testicles;
orchis, = a kind of olive of an oblong shape; another plant, so called because of the shape of its roots, the orchis, apparently shaped like a testicle.
orchis, = orchid (Pliny);
Orchideæ (Linnaeus), the plant's family name, from Latin orchis, a kind of orchid, from Greek orkhis (genitive orkheos) "orchid," literally "testicle," from orghi-, the standard root for "testicle".

(referring to the paired testicle shaped tubers of the European genus (herba orchis). Since antiquity it has been regarded as an aphrodisiac)
(LS, ld, Ca, I7, K3)

orobos, = the vetch;
ancho, = strangle, to throttle.

(the genus Orobanche is parasitic and believed to kill the host plant)

(LS, BL, Ox, Le)

For Osmund**, the waterman or for the Anglo-Saxon equivalent of Thor, god of thunder.
(Ro)

oxys, = sharp, keen, pointed;
(sharp, keen, shrill, pungent, acid)
-alis, = belonging to, resembling, provided with, pertaining to.
(oxalic acid content)
(LS, BL, Le)