This paper applies the overlapped subaperture algorithm (OSA) for geosynchronous (GEO) spaceborne/missile-borne bistatic SAR (GEO SMB-BISAR) imaging. Compared with the original bistatic SAR, GEO SMB-BISAR performs better in flexibility and security. The essence of the OSA is to perform the imaging process twice by azimuth subaperture division. The quadratic phase errors (QPE) induced by the planar wavefront assumption can be compensated by two steps imaging processing within and between subapertures. The OSA can expand a wider imaging swath than the polar format algorithm (PFA). Validation results demonstrate the validity of the OSA methodology.