(single-coated)

OM-1 with mirror lockup

Vignetting = B
Distortion = slight pincushion (none?)
Aperture Center Corner
f/2.8    C+     C+
f/4      B-     B-
f/5.6    B+     B+
f/8      B+     B+
f/11     A      A-
f/16     A-     B
Notes: Moderately low contrast at f/16, moderate contrast elsewise.

OM-4 with mirror and aperture prefire, average of two samples
Vignetting = C @ f/2.8, B+ @ f/4, A- @ f/5.6
Distortion = slight waveforming with slight barrel
Aperture Center Corner
f/2.8    A-     B/B-
f/4      A      B+
f/5.6    A      A-
f/8      A-     A
f/11     A/A-   A-
f/16     B+     A-/B+
Notes: Different lens samples than the OM-1 test above. Moderate contrast @ f/2.8; high contrast @ f/5.6 to f/11; moderately low contrast @ f/16.