Bellows are expensive and take a lot of fiddling to get to work right. But they provide the highest magnification, the widest range of magnification, and can adapt almost any lens.
Reversed lenses are a pita. Proper exposure and focus are achieved by trial and error. They are most useful when used in conjunction with a macro lens, a bellows, and when you want more than 1:1.
Extension tubes work well, and are not very expensive for the OM system. Especially if you have the lauded 50mm f/3.5, they will provide the most bang-for-the-buck and provide almost as much versatility as does a bellows unit. They are slightly less stellar with non-macro lenses, but still work just fine. If you expose manually, pay attention to the extension corrections (this applies to bellows as well.)
Auxiliary close-up lenses are the easiest to use, and range from dirt cheap to quite costly. Expensive cu lenses provide better edge-to-edge sharpness; cheap works just fine in the center, especially in B&W so you don't get any color fringing issues. They're no bigger than a filter, so you can carry them anywhere. They will attach to any lens, as long as the thread size is right, or you have the appropriate stepper rings. There is no 'extension penalty' when using aux lenses.