| O'danielite | H₂NaZn₃(AsO₄)₃ | Monoclinic | 3.0 | — |
| Oberthürite | — | Cubic | — | — |
| Oboniobite | Mg₄Nb₂O₉ | Trigonal | — | 4.35 g/cm³ |
| Oboyerite | H₆(Pb,Ca)₆(Te₄+O₃)₃(Te₆+O₆)₂ * 2H₂O | — | — | — |
| Obradovicite-KCu | [K₂(H₂O)₁₇Cu(H₂O)₆][Mo₈As₂Fe³⁺₃O₃₄(OH)₃] | Orthorhombic | — | — |
| Obradovicite-NaCu | [Na₂(H₂O)₁₇Cu(H₂O)₆][Mo₈As₂Fe³⁺₃O₃₄(OH)₃] | Orthorhombic | — | — |
| Obradovicite-NaNa | [Na₂(H₂O)₁₆Na(H₂O)₆][Mo₈As₂Fe³⁺₃O₃₃(OH)₄] | Orthorhombic | — | — |
| Odigitriaite | CsNa₅Ca₅[Si₁₄B₂O₃₈]F₂ | Monoclinic | — | — |
| Odinite | (Fe,Mg,Al,Fe,Ti,Mn)₂.₄((Si,Al)₂O₅)(OH)₄ | Monoclinic | — | — |
| Odintsovite | K₂Na₄Ca₃Ti₂Be₄Si₁₂O₃₈ | Orthorhombic | — | — |
| Oenite | CoSbAs | Orthorhombic | — | — |
| Offretite | KCaMg(Si₁₃Al₅)O₃₆·15H₂O | Hexagonal | 4.0 | — |
| Oftedalite | KSc₂☐₂Be₃Si₁₂O₃₀ | Hexagonal | 6.0 | — |
| Ogdensburgite | Ca₂Fe³⁺₄(Zn,Mn²⁺)(AsO₄)₄(OH)₆·6H₂O | — | — | — |
| Ognitite | NiBiTe | Trigonal | — | — |
| Ohmilite | Sr₃(Ti,Fe³⁺)(Si₂O₆)₂(O,OH)·2H₂O | Monoclinic | — | — |
| Ohtaniite | Mg₃(Si₀,₅◻₀,₅)Si₂O₈ | Orthorhombic | — | — |
| Ojuelaite | ZnFe³⁺₂(AsO₄)₂(OH)₂·4H₂O | Monoclinic | 3.0 | — |
| Okanoganite-(Y) | (Na,Ca)₃(Y,Ce)₁₂Si₆B₂O₂₇F₁₄ | — | — | — |
| Okayamalite | Ca₂B(BSiO₇) | Tetragonal | — | — |
| Okenite | Ca₁₀Si₁₈O₄₆·18H₂O | Triclinic | — | — |
| Okhotskite | Ca₂Mn²⁺Mn³⁺₂(Si₂O₇)(SiO₄)(OH)₂·H₂O | Monoclinic | — | — |
| Okieite | Mg₃[V₁₀O₂₈]·28H₂O | Triclinic | — | — |
| Okruginite | Cu₂SnSe₃ | Monoclinic | — | — |
| Okruschite | Ca₂Mn²⁺₅Be₄(AsO₄)₆(OH)₄·6H₂O | Monoclinic | 3.5 | — |
| Oldhamite | CaS | Cubic | 4.0 | 2.58 g/cm³ |
| Oldsite-(K) | K₂Fe²⁺[(UO₂)(SO₄)₂]₂(H₂O)₈ | Orthorhombic | — | — |
| Olekminskite | Sr(Sr,Ca,Ba)(CO₃)₂ | Trigonal | 3.0 | — |
| Olenite | Na(Al₃)Al₆(Si₆O₁₈)(BO₃)₃O₃(OH) | Trigonal | 7.0 | — |
| Olgafrankite | Ni₃Ge | Cubic | — | 9.39 g/cm³ |
| Olgite | Na(Na,Sr)₂Ba(PO₄)₂ | Trigonal | — | — |
| Olivenite | Cu₂(AsO₄)(OH) | Orthorhombic | 3.0 | — |
| Olkhonskite | (Cr,V)₂Ti₃O₉ | Monoclinic | — | — |
| Olmiite | CaMn[SiO₃(OH)](OH) | Orthorhombic | — | — |
| Olmsteadite | KFe²⁺₂(Nb₅+,Ta₅+)(PO₄)₂O₂ * 2H₂O | — | 4.0 | — |
| Olsacherite | Pb₂(SeO₄)(SO₄) | Orthorhombic | — | — |
| Olsenite | KFe₄(PO₄)₃ | Orthorhombic | — | — |
| Olshanskyite | Ca₂[B₃O₃(OH)₆]OH·3H₂O | Triclinic | — | — |
| Olympite | LiNa₅(PO₄)₂ | — | 4.0 | — |
| Omariniite | Cu₈Fe₂ZnGe₂S₁₂ | Orthorhombic | — | — |
| Omeiite | (Os,Ru)As₂ | Orthorhombic | 7.0 | — |
| Ominelite | (Fe²⁺,Mg)(Al,Fe³⁺)₃(SiO₄)(BO₃)O₂ | Orthorhombic | — | — |
| Omongwaite | Na₂Ca₅(SO₄)₆·3H₂O | Monoclinic | — | — |
| Omphacite | (Ca,Na)(Mg,Fe,Al)Si₂O₆ | Monoclinic | — | — |
| Omsite | Ni₂Fe³⁺(OH)₆[Sb(OH)₆] | Trigonal | — | — |
| Ondrušite | CaCu₄(AsO₄)₂(AsO₃OH)₂·10H₂O | Triclinic | — | — |
| Oneillite | Na₁₅Ca₃Mn₃Fe²⁺₃Zr₃Nb(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂ | Trigonal | — | — |
| Onoratoite | Sb₈Cl₂O₁₁ | — | — | — |
| Oosterboschite | (Pd,Cu)₇Se₅ | Orthorhombic | — | — |
| Ootannite | Th⁴⁺₂W⁶⁺₄O₁₆·5H₂O | Monoclinic | — | — |
| Opal | SiO₂·nH₂O | — | — | — |
| Ophirite | Ca₂Mg₄[Zn₂Mn³⁺₂(H₂O)₂(Fe³⁺W₉O₃₄)₂]·46H₂O | Triclinic | — | — |
| Oppenheimerite | Na₂(UO₂)(SO₄)₂·3H₂O | Triclinic | — | — |
| Orcelite | Ni₅-xAs₂, x ~ ₀.₂₅ | Hexagonal | — | — |
| Ordoñezite | ZnSb⁵⁺₂O₆ | Tetragonal | — | — |
| Oregonite | Ni₂FeAs₂ | Hexagonal | 5.0 | — |
| Oreillyite | Cr₂N | Trigonal | — | — |
| Organovaite-Mn | K₂MnNb₄(Si₄O₁₂)₂O₄·5-7H₂O | — | — | — |
| Organovaite-Zn | K₂Zn(Nb,Ti)₄(Si₄O₁₂)₂(O,OH)₄·6H₂O | — | — | — |
| Orickite | ₂CuFeS₂ * H₂O | Hexagonal | — | — |
| Orientite | Ca₈Mn³⁺₁₀(SiO₄)₃(Si₃O₁₀)₃(OH)₁₀·4H₂O | Orthorhombic | — | — |
| Orishchinite | Ni₂P | Orthorhombic | — | — |
| Orlandiite | Pb₃Cl₄(Se⁴⁺O₃)·H₂O | — | — | — |
| Orlovite | KLi₂TiSi₄O₁₁F | Monoclinic | — | — |
| Orlymanite | Ca₄Mn²⁺₃Si₈O₂₀(OH)₆·2H₂O | Hexagonal | — | — |
| Orpiment | As₂S₃ | Monoclinic | — | — |
| Orschallite | Ca₃(S⁴⁺O₃)₂(SO₄)·12H₂O | — | — | — |
| Orthobrannerite | U⁴⁺U⁶⁺Ti₄O₁₂(OH)₂ | Orthorhombic | — | — |
| Orthoclase | K(AlSi₃O₈) | Monoclinic | 6.0 | 2.46 g/cm³ |
| Orthocuproplatinum | Pt₃Cu | Orthorhombic | — | — |
| Orthogersdorffite | NiAsS | Orthorhombic | — | 6.02 g/cm³ |
| Orthojoaquinite-(Ce) | NaBa₂Ce₂FeTi₂[Si₄O₁₂]₂O₂(O,OH)·H₂O | Orthorhombic | — | — |
| Orthojoaquinite-(La) | NaBa₂La₂Fe²⁺Ti₂[Si₄O₁₂]₂O₂(O,OH)·H₂O | Orthorhombic | — | — |
| Orthominasragrite | (V⁴⁺O)(SO₄)·5H₂O | Orthorhombic | — | — |
| Orthopinakiolite | Mg₂Mn³⁺O₂(BO₃) | Orthorhombic | — | — |
| Orthoserpierite | Ca(Cu,Zn)₄(SO₄)₂(OH)₆·3H₂O | Orthorhombic | — | — |
| Orthowalpurgite | (BiO)₄(UO₂)(AsO₄)₂·2H₂O | Orthorhombic | — | — |
| Osakaite | Zn₄(SO₄)(OH)₆·5H₂O | Triclinic | — | — |
| Osarizawaite | CuPbAl₂(SO₄)₂(OH)₆ | Trigonal | — | — |
| Osarsite | OsAsS | Monoclinic | — | — |
| Osbornite | TiN | Cubic | — | 5.39 g/cm³ |
| Oscarkempffite | Ag₁₀Pb₄(Sb₁₇Bi₉)S₄₈ | Orthorhombic | — | — |
| Oskarssonite | AlF₃ | Trigonal | — | 3.26 g/cm³ |
| Osmium (native) | Os | Hexagonal | — | — |
| Osumilite | (K,Na)(Fe²⁺,Mg)₂(Al,Fe³⁺)₃(Si,Al)₁₂O₃₀ | Hexagonal | — | — |
| Osumilite-(Mg) | KMg₂Al₃(Al₂Si₁₀)O₃₀ | Hexagonal | 6.5 | — |
| Oswaldpeetersite | (UO₂)₂(CO₃)(OH)₂·4H₂O | Monoclinic | — | — |
| Otavite | CdCO₃ | Trigonal | — | 4.96 g/cm³ |
| Otjisumeite | PbGe₄O₉ | Triclinic | 3.0 | 5.77 g/cm³ |
| Ottemannite | Sn₂S₃ | Orthorhombic | — | — |
| Ottensite | Na₃(Sb₂O₃)₃(SbS₃)·3H₂O | Hexagonal | — | 4.14 g/cm³ |
| Ottohahnite | Na₆(UO₂)₂(SO₄)₅(H₂O)₇·1.5H₂O | Triclinic | — | — |
| Ottoite | Pb₂TeO₅ | Monoclinic | — | 8.74 g/cm³ |
| Ottrélite | — | — | — | — |
| Otwayite | Ni₂(CO₃)(OH)₂·H₂O | Orthorhombic | — | — |
| Oulankaite | (Pd,Pt)₅(Cu,Fe)₄SnTe₂S₂ | Tetragonal | — | — |
| Ourayite | Ag₃Pb₄Bi₅S₁₃ | — | — | — |
| Oursinite | Co(UO₂)₂(SiO₃OH)₂·6H₂O | — | — | — |
| Ovamboite | Cu₁₀Fe₃WGe₃S₁₆ | — | — | — |
| Overite | CaMgAl(PO₄)₂(OH)·4H₂O | Orthorhombic | — | — |
| Owensite | (Ba,Pb)₆(Cu,Fe,Ni)₂₅S₂₇ | Cubic | — | — |
| Owyheeite | Ag₃Pb₁₀Sb₁₁S₂₈ | Monoclinic | — | — |
| Oxammite | (NH₄)₂(C₂O₄)·H₂O | Orthorhombic | 2.5 | 1.50 g/cm³ |
| Oxo-magnesio-hastingsite | NaCa₂(Mg₂Fe³⁺₃)(Si₆Al₂)O₂₂O₂ | Monoclinic | 6.0 | — |
| Oxo-mangani-leakeite | NaNa₂(Mn³⁺₄Li)Si₈O₂₂O₂ | Monoclinic | — | — |
| Oxy-chromium-dravite | NaCr₃(Cr₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O | Trigonal | 7.5 | — |
| Oxy-dravite | Na(Al₂Mg)(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃O | Trigonal | 7.0 | — |
| Oxy-foitite | ☐(Fe²⁺Al₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O | Trigonal | 7.0 | — |
| Oxy-schorl | Na(Fe²⁺₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O | Trigonal | 7.0 | — |
| Oxy-vanadium-dravite | Na(V)₃(V₄Mg₂)Si₆O₁₈(BO₃)₃(OH)₃O | Trigonal | 7.5 | — |
| Oxybismutomicrolite | (Bi₁.₃₃☐₀.₆₇)Ta₂O₆O | Cubic | — | — |
| Oxycalciomicrolite | Ca₂Ta₂O₇ | Cubic | — | — |
| Oxycalciopyrochlore | Ca₂Nb₂O₆O | Orthorhombic | 5.0 | 4.41 g/cm³ |
| Oxycalcioroméite | Ca₂Sb₂O₆O | Cubic | — | 5.16 g/cm³ |
| Oxykinoshitalite | BaMg₂Ti⁴⁺O₂(Si₂Al₂)O₁₀ | Monoclinic | — | — |
| Oxynatromicrolite | (Na,Ca,U)₂(Ta,Nb)₂O₆(O,F) | Cubic | — | — |
| Oxyphlogopite | K(Mg,Ti,Fe)₃[(Si,Al)₄O₁₀](O,F)₂ | Monoclinic | — | — |
| Oxyplumbopyrochlore | Pb₂Nb₂O₆O | Cubic | — | — |
| Oxyplumboroméite | Pb₂Sb₂O₇ | Cubic | 5.0 | 6.73 g/cm³ |
| Oxystannomicrolite | Sn₂Ta₂O₆O | Cubic | — | — |
| Oxystibiomicrolite | (Sb³⁺,Ca)₂Ta₂O₆O | Cubic | 5.5 | — |
| Oxyvanite | V³⁺₂V⁴⁺O₅ | Monoclinic | — | — |
| Oxyyttrobetafite-(Y) | Y₂Ti₂O₆O | Cubic | 5.0 | — |
| Oyelite | Ca₁₀Si₈B₂O₂₉ * 12.₅H₂O | Orthorhombic | — | — |
| Oyonite | Ag₃Mn₂Pb₄Sb₇As₄S₂₄ | Monoclinic | — | — |
| Ozernovskite | — | Monoclinic | — | — |
| Ozerovaite | Na₂KAl₃(AsO₄)₄ | Orthorhombic | — | — |
| Örebroite | — | — | — | — |