With an annual production of 1500 metric tons of shucked oysters, Willapa Bay, WA currently produces more oysters than any other estuary in the United States. This production is mainly composed of the Pacific oyster (Crassostrea gigas), rather than the native Olympia oyster (Ostrea lurida). Historically, Willapa Bay was home to vast Olympia oyster beds, which formed the foundation of a major extractive fishery in the late 1800s. Yet the historical baseline extent of this habitat is poorly understood as it was first documented following decades of exploitation and was therefore based on a shifted baseline. An extensive and thorough literature review was undertaken to ascertain whether oyster beds mapped as “cultivated” in the 1890s were in fact originally wild beds. The most complete harvest statistics to date have been presented for the Olympia oyster in Willapa Bay (from 1849 to 2011) to provide useful historical insights into the expansion and collapse of the Olympia oyster fishery and discuss the evidence for numerous drivers of decline. Compelling evidence found that the historical extent of oyster beds in Willapa Bay may have been as great as 9774 ha, or 27% of the bay bottom, equating to a standing oyster biomass on the order of 3.6 × 10⁶kg. This figure is significantly greater that the often quoted ∼2600 ha, derived if only beds marked as natural in the 1880s and 1890s are considered part of the original extent.