field observation -> field inventory and monitoring (228) earthquake occurrence maps (2) -> earthquake occurrences -> maps and atlases earthquake occurrences (3) ocean bottom characteristics -> sea-floor characteristics (1) ocean characteristics (1) ocean floor features -> seafloor features (1) ocean mixed-layer depth -> ocean characteristics (1) ocean surface characteristics -> ocean characteristics (1) offices -> buildings (9) official communications (2) lists of publications -> catalogs and indexes (4) improvement of scientific data usability (1) collection of specimens -> specimen collecting (3) bodies of water -> hydrographic features (18) oil fields (1) oilfields -> oil fields (1) olive oil mills -> industrial sites (9) oil refineries -> industrial sites (9) oil resources (1) oil wells -> wells (5) olive oil mills -> industrial sites (9) open pit mines -> mine sites (167) optically-stimulated luminescence dating -> luminescence dating (1) optical microscopy (2) ore deposits -> mineral deposits (155) ore deposits (metallic) -> metallic mineral resources (193) ore formation (23) metallic ores -> metallic mineral resources (193) ore treatment plants -> industrial sites (9) dissolved organic compounds (3) organic decomposition -> biogeochemical cycling (3) organism groupings (non-taxonomic) (8) biological organisms -> organisms (6) organisms (6) orienteering -> geolocation measurement (1) osmium (23) outfall -> contamination and pollution (96) overfalls -> hydrographic features (18) intermittent oxbow lakes -> lakes (4) oxbow lakes -> lakes (4) oxidation and reduction (1) dissolved oxygen -> oxygen content (water) (10) oxygen (10) oxygen content (water) (10) oxygen isotope analysis (10)