You can download a manual from this site:
http://www.heier.com/olydocs/
Note; it is a very slow download.
I am assuming the lenses you mention are the Zuiko (Olympus) lenses, and will comment based on that assumption. There were variations of the 28mm. I had the slow F3.5. It was never one of my favourites, but other people like it better than me.
Firstly, you are right that 100 ISO is too slow for indoor shots. You should be able to use it on cloudy days, but only at wider apertures (That is closer to the F4 on the zoom).
Although I eventually moved towards the 85 F2 and 100 F2.8 prime lenses, I was addicted to the zoom when I was travelling for 15 months in 1978-1979. I did not find the zoom to produce dull shots that were not crisp. Others may differ.
http://www.pbase.com/zuiko40/rtw
Not all of those shots were with the zoom, but some of them were.
In terms of the dullness. Is the meter accurate? The OM-1 uses mercury batteries no longer available, at least not in North America. The substitutes may be giving you a reading that is underexposure.
On the other hand, the meter may be fooled by the subject matter. Meters think everything is supposed to be medium grey. So if you have a lot of sky in the picture, you might want to take a reading of the area away from the sky, or slightly underexpose the picture. On the other hand, when you are exposing for dark rocks, you might want to try underexposing a bit to hold some colour in the sky.
If there is a big difference in brightness in different areas of the picture, than the film just may not be able to hold it.
I don't know if you are using print film or slide film. It could be the result of the printing, and there may be some detail there after all. I used to do my own printing in pre-Photoshop days, and sometimes you could recover detail and colour in the sky by burning it in (giving more exposure in the printing process to that part of the picture.) But Photoshop is better.
Anyway, please follow up with more questions.
Also, if you decided to take the camera to Switzerland, I would look around for a used 50 mm 1.8 lens. There should be tons of them around in the bigger cities, because the OM-1 was a popular camera, and that was the "kit" lens in pre-zoom days.
In terms of the lack of sharpness, a good starting rule for hand-holding a camera is to use a shutter-speed one over the lens length, at the very least. For example, with the 28mm, 1/30. With the zoom at the short end, 1/60 and at the longer end 1/125.
The best apertures for the lenses, in terms of sharpness, is probably around F8 or F11.
And don't use junk film.
Please feel free to ask follow up questions.