Original type designation O. rosula by Lyman (1865). H.L. Clark (1915) synonymized O. rosula with O. pentaphyllum (Pennant, 1777) which may have caused later authors to accept the latter name as type species. A.M. Clark (1967) proposed to suppress pentaphyllum in favour of the more widely applied name fragils. That proposal seems never to have been decided and technically, O. pentaphyllum has priority. See also under species O. fragilis.