(single-coated)

Vignetting = B
Distortion = slight barrel

OM-1 with mirror lockup
Aperture Center Corner
f/2.8    D-     C-
f/4      C      C
f/5.6    B      B
f/8      B+     A-
f/11     A-     A-
f/16     A      A-
Notes: Moderately low contrast at f/2.8 and f/16, moderate contrast elsewise but high resolution.

OM-4T with mirror and aperture prefire; same lens as OM-1 based test
Aperture Center Corner
f/2.8    B      C+
f/4      A-     B-
f/5.6    A      A-
f/8      A-     A-
f/11     A-     B+
f/16    A-      A-
Notes: Moderately low contrast images at f/2.8; moderate contrast images at f/4, f/11 and f/16; moderately high contrast images at f/5.6 and f/8.