Families (As per The Plant List)

 
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | A-Z

O

Oleandraceae - 1 (Oleandra musifolia)

Ophioglossaceae - 9 (Botrychium daucifolium, Botrychium lanuginosum, Helminthostachys zeylanica, Ophioglossum costatum, Ophioglossum gramineum, Ophioglossum nudicaule, Ophioglossum parvifolium, Ophioglossum pedunculosum, Ophioglossum reticulatum)

Osmundaceae - 2 (Osmunda hugeliana, Osmunda regalis)