This is most likely a shutter-curtain problem, not a film or development issue. On the OM-1’s mechanical focal-plane shutter, the first and second curtains travel across the film. At fast shutter speeds, they form a narrow moving slit. If one curtain is delayed, dragging, or out of adjustment, part of the frame gets blocked and you see a dark band or black edge.

That matches your symptoms: in bright sunlight the camera uses faster shutter speeds, so the problem shows up more clearly. Indoors, slower speeds are used, and the shutter has enough time to fully open, so the issue may not appear.

The fix is typically a CLA/service or shutter repair. If the camera is otherwise in good shape, a technician can inspect and adjust it. If repair cost is high, some people choose to replace the body instead.