300mm

I was thinking early on about getting a longish telephoto lens but it's taken now to find an affordable quality option for my OM10 and 40 bodies as most of the independent options from the late 1970's and 80's were rather so-so at best and you are normally talking of around £100 upward for Olympus's own.

Shopping around I found something that would work, the Tamron Adaptall 300mm F5.6 which was originally introduced  in 1976 and phased out in 1979 for the later Adaptall 2  close focusing version.
The early Adaptall lenses do take the later mounts and although this as bought came mounted for the old M42 screw fitting, changing to Olympus OM is quick and easy and for £36 was a bargain.

Technical notes

Lens construction: four group 4 elements

Minimum aperture: 22

Minimum focusing distance: 2.5m

Filter diameter: 58mm

Weight: 580g