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| O'Hara Cr. (1) | O'Hara Creek (1) | O'Shea Creek (3) |
| Oak Creek (9) | Oakcreek (Or.) (1) | Oakland (1) |
| Oakland's Fish Camp (1) | Oakland's Marina (2) | Oar Creek (3) |
| ocean acidification (19) | ocean acidification, (1) | Oceanic (161) |
| Oceanlake (2) | Oceanlake (Or.) (1) | oceanography (3) |
| Oceanside (1) | Oceanside (Or.) (4) | Oceanside (Oregon) (1) |
| Oceanside Littoral Cell (2) | Ocmulgee River (1) | Odonthalia floccosa (1) |
| Oean acidification (1) | Ohanapecosh Formation (1) | Oil (95) |
| oil spills (5) | Oithona similis (1) | Okanogan River (2) |
| Okie Creek (1) | Oklahoma (12) | Okpikruak Formation (1) |
| Olalla (Or.) (1) | Olalla Creek (23) | Olalla Reservoir (Yaquina River) (1) |
| Olalla Slough (12) | Old Mill Site Riffle (1) | Oldham Creek (3) |
| oldsquaw=Clangula hyemalis (1) | Oligocene (84) | Oligochaeta (2) |
| Olivella biplicata (1) | Oliver Creek (1) | Ollala Slough (1) |
| Ollalla Slough (1) | Olssonville (1) | Olympia oyste = Ostrea lurida (Ostrea conchaphila) (3) |
| Olympia oyster = Ostrea conchaphila (Ostrea lurida) (13) | Olympia oyster = Ostrea lurida (11) | Olympia oyster = Ostrea lurida (O. conchaphila) (2) |
| Olympia oyster = Ostrea lurida (Ostrea conchaphila) (7) | Olympia oyster = Ostrea lurida [Ostrea conchaphila] (1) | Olympia oyster=Ostrea lurida (Ostrea conchaphila) (91) |
| Olympia oyster=Ostrea lurida (Ostrea conchaphila),Netarts Bay, Whiskey Creek, Pacific Coast, aquatic invertebrates, mollusks, bivalves, reproduction, recruitment, habitats, salinity, water temperature, chlorophyll, human impacts, geographical distribution (1) | Olympic mudminnow = Novumbra hubbsi (1) | Olympic oyster = Ostrea conchaphila (Ostrea lurida) (1) |
| Oman (1) | Omar Formation (1) | Ona Beach (13) |
| Ona Beach State Park (2) | Oncorhynchus species (4) | Oncorhynchus spp. (11) |
| Oneatta (Or.) (1) | Oneatta Point (35) | Ono formation (1) |
| opaleye=Girella nigricans (1) | Open Bay Formation (1) | Opossum shrimp = Neomysis mercedis (1) |
| Or. (4) | Or.: Winchester (1) | Orconectes neglectus = ringed crayfish (1) |
| Oregon (437) | Oregon Aqua Foods (10) | Oregon Aqua-Foods (35) |
| Oregon chub = Oregonichthys crameri (5) | Oregon chub=Hybopsis crameri (3) | Oregon chubs = Oregonichthys spp. (1) |
| Oregon Coast (8) | Oregon Coast Aquarium (3) | Oregon Coast Aquarium mudflat (1) |
| Oregon Coast Range (2) | Oregon Coast Range, Coho salmon = Oncorhynchus kisutch, Coos Bay Watershed, Coquille River Basin, Nestucca River Basin, agriculture, environmental law and policy, hydrography, logging, natural resource management, riparian areas, terrestrial vegetation (1) | Oregon Coast Range, environmental law and policy, logging, agriculture, riparian areas, terrestrial vegetation (1) |
| Oregon Coast, Pacific salmonids = Oncorhynchus spp., Coho salmon = Oncorhynchus kisutch, climate change, depleted populations, ecosystem health, endangered species, habitats, hatcheries, human impacts, life history, ocean acidification, water temperatur (2) | Oregon Coastal Conservation and Development Commission (1) | Oregon Cooperative Wildlife Research Unit (1) |
| Oregon Dunes (1) | Oregon Dunes National Recreational Area (1) | Oregon Dunes Recreational Area (1) |
| Oregon Fish Commission (3) | Oregon Fish Commission Research Laboratory (6) | Oregon gumplant = Grindelia stricta (2) |
| Oregon Hatchery Research Center (2) | Oregon Oyster Company (2) | Oregon Oyster Farm (5) |
| Oregon Oyster Farms (6) | Oregon pikeminnow = Ptychocheilus oregonensis (3) | Oregon shore crab = Hemigrapsus oregonensis (13) |
| Oregon silverspot butterfly = Speyeria zerene Hippolyta, Early blue violet = Viola adunca, Cascade Head, Rock Creek-Big Creek, Bray Pt., and Mt. Hebo, Del Norte Conservation Area (Lake Earl), depleted populations, threatened species, habitats, human imp (1) | Oregon slender salamander = Batrachoseps wrighti (1) | Oregon State Game Commission (1) |
| Oregon State University (3) | Oregon State University Hatfield Marine Science Center (148) | Oregon, environmental law and policy, land ownership, tidelands, beaches, Beach Bill (1) |
| Oregon, Pacific Northwest, Pacific Southwest, Intermountain West, water temperature, climate, environmental monitoring, mathematical modeling (1) | Orford Reef (4) | organic carbon (14) |
| organic matter (1) | orientation (1) | Orthione griffenis (3) |
| osmosis (1) | Osprey = Pandion halioetus (2) | OSU Swanson Aquaculture Research Station (1) |
| Otadai Formation (1) | Otis (Or.) (5) | otoliths (1) |
| Otter Creek (2) | Otter Crest (5) | Otter Point Formation (1) |
| Otter Rock (14) | Otter Rock Marine Reserve (1) | Otter Slough (4) |
| outfall (14) | Oxbow Creek (tributary of the Siuslaw River) (5) | oxygen (12) |
| Oxytrema silicula (3) | Oyster Bay (Wa.) (1) | Oyster City (1) |
| oysters (24) | Oysterville (34) | Oysterville (Or.) (3) |
| Oysterville (Wa.) (2) | ozone (1) | O’Hara Creek (1) |
