There was a recent "discussion" on another thread relative to a choice between the Olympus 300mm prime and the 100-400mm zoom. I think the OP's final decision was to keep the prime. I have both and if I had to choose, I would keep the 100-400mm for its versatility. I took the 300mm lens out today in sub-freezing weather and was happy to find some wild Mute Swans in a nearby river. More often than not, their heads are under the murky water looking for edible plant life, and making their heads and necks yellower than the rest of their bodies. Although the 300mm lens served me fine for these shots, I missed other opportunities in my walk along the river due to the fixed focus length. To each her or his own.
OM-1OLYMPUS M.300mm F4.0 lens300mmf/4.01/250s200 ISO0.0 EV
OM-1OLYMPUS M.300mm F4.0 lens300mmf/4.51/200s200 ISO0.0 EV
OM-1OLYMPUS M.300mm F4.0 lens300mmf/4.51/200s200 ISO0.0 EV