Forestry residue-source oil-tea cake (consisting of plant protein, tannin, carbohydrate compounds, and tea saponin) is a major by-product from the pressing process of Camellia (Camellia oleifera Abel) seeds. In this study, we evaluated oil-tea cake as a potential extender to replace the current industry extender, wheat flour, for applications with phenol-formaldehyde (PF) plywood adhesives. We evaluated mixing properties, curing characteristics, and adhesion performance of the adhesive containing oil-tea cake as compared with those of the standard wheat flour–based adhesive. The oil-tea cake–based adhesive showed excellent mixing properties and working life, and its viscosity met the recommended value for the sprayline coater. The adhesive containing oil-tea cake was comparable in wet shear strength and wood failure values of plywood to the wheat flour–based adhesive. Thermal analysis showed that adhesives containing oil-tea cake had significant curing reactions because of the relatively large curing enthalpy (DH) value of oil-tea cake. These results indicate that oil-tea cake is suitable as a low-cost and effective plywood adhesive extender. Therefore, using oil-tea cake is economically beneficial because it not only makes use of what would otherwise be forestry residue, but it is cheaper to use as a plywood adhesive than wheat flour.