Olsacherite Pb2(SeO4)(SO4)
OpalHydrophane - porous
Hyalite - glassy
Prase - green
Precious opal - opalescent
Fire Opal - red
SiO2·n(H2O)
Orpiment As2S3
Orthoserpierite Ca(Cu,Zn)4(SO4)2(OH)6·3(H2O)
Osarizawaite PbCuAl2(SO4)2(OH)6
Otavite CdCO3
Oyelite Ca10B2Si8O29·12(H2O)