Try to make your search as simple as possible:
Onosma will find all items including that sequence of letters.
Search results for "Onosma":
"Onosma coerulea" - Moltkia coerulea
"Onosma rigida" - Onosma taurica (name unresolved)
"Onosma rupestris" - Onosma tenuiflora
"Onosma taurica (unresolved)" - Onosma rigida
"Siphonosmanthus delavayi" - Osmanthus delavayi