Effects of alkali washing were determined after steam treatment of Betung bamboo strands relative to the properties of the bamboo oriented strand boards (BOSBs). The strands were subjected to steam treatment at 126 °C for 1 h under 0.14 MPa of pressure, followed by washing with sodium hydroxide solution at concentrations ranging from 1% to 5% for 30 s. Three-layer BOSBs were manufactured with a target density of approximately 0.7 g/cm3 using 8% phenol formaldehyde resin with the addition of 1% wax. The shelling ratio of the BOSBs was set to be 1:1:1. The physical and mechanical properties increased significantly with the alkali washing treatment at concentrations from 1% to 3%, and the greatest properties were obtained by washing at 3% alkali concentration. However, the washing treatment at concentrations of 4% and 5% reduced the physical and mechanical properties of the BOSBs.