The Zuiko zoom to get is the 35-80 f2.8. Perhaps the last OM Zuiko lens that was designed, it is probably the sharpest medium range zoom made by anyone at the time and maybe even now. Outstanding sharpness, color rendition, excellent bokeh and a bright, constant f2.8 aperture. I recall Popular Photography reviewed this lens and found it to be the best performing zoom they had tested in its general zoom range. The bayonet lens hood also is great. One of my most used OM Zuikos (I have 17 of them). The image quality is every bit as good as excellent primes in its zoom range.
Of course, the lens has some downsides. First the price -- today goes for over $1,000 typically for a good condition sample. Second, the lens is larger and heavier than the typical OM Zuiko. Third, despite being very well built, when the lens is full extended and shaken, there is a slight amount of play that can be felt in the front of the lens. I understand that this is normal for this lens (as confirmed by John Hermanson of Camtech and other things I have read) and does not affect image quality at all.
In this general range, I also have a Vivitar Series 1 28-90 and Tamron SP 35-80. They are also excellent and provide sharp images. However, in comparing them by image quality, the Zuiko 35-80 is the best, followed by the Tamron SP 35-80 and then the Series 1 28-90. The Tamron SP does focus the closest though and has outstanding closeup image quality and is the most compact and by far the lightest. Given that the latter two lenses cost a mere fraction of the cost of the Zuiko, I do use them quite a bit where they might be exposed to some less than ideal conditions.