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

O

OLEANDRACEAE - 1 ( Oleandra musifolia)

OPHIOGLOSSACEAE - 14 ( Botrychium daucifolium, Botrychium lanuginosum, Helminthostachys zeylanica, Ophioglossum costatum, Ophioglossum eliminatum, Ophioglossum gramineum, Ophioglossum lancifolium, Ophioglossum lusitanicum, Ophioglossum oleosum, Ophioglossum parvifolium, Ophioglossum pendulum, Ophioglossum petiolatum, Ophioglossum polyphyllum, Ophioglossum reticulatum)

OSMUNDACEAE - 3 ( Lygodium longifolium, Lygodium salicifolium, Osmunda hilsenbergii)