75-150 Zoom

For more years than I really care for  arguments have raged over the qualities of lenses that cover several focal lengths  over fixed focal length ones.

Olympus themselves had manufactured a good number during what might in hindsight  be called the "Golden Era" of the OM system  in the late 1970's and 1980's that met their exacting standards.
Just what is the argument against them?
I think the first is for serious landscape photography they are harder to use with filters, specifically those square ones like Cokin's that screw on the filter thread of a lens because most zoom lenses have front elements that rotate when you focus whereas on fixed length ones they do not.
The other is during that era many cheaper zoom lenses came on the market that weren't so well designed spoiling their image. 
It was said at the time you needed to spend two to three times as much for a zoom lens of a similar quality to two or more fixed length ones and many amateurs just were not prepared to spend that.
This said, a good one has real uses like being able to easily control the final image you take very precisely not least on transparency film and this example I own, the 75-150mm F4 is a good example as it provides a very high image quality, offers a wide range of telephoto lengths and yet weighs little more than single 200mm lens which is comfortable for hand hold usage. 
I find its coverage is a good match for what I want in the field saving me carrying the 90mm and 135 and rarely need the 200mm as it's top end of 150 is adequate while saving me from changing lenses frequently.
It also uses a 49mm filter thread just like the regular standard and wide angle lenses which makes working with polarizers and square filters more straightforward for not having to double up on attachment rings and screw in filters.
As the OM system in the early 1990's moved very much toward  advanced amateurs and professionals with the OM4Ti being the only 'new' body available, while only a  few fixed length lenses went from the lens range such as the 135mm, all the Zooms did until they introduced the 35-80 F2.8 in 1995 issued with the OM3Ti.

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

200mm

200mm lens are not the sort of thing you use everyday that's for sure but they have their place in the photographic scheme of thing such as Landscapes - mountain images - where their stacked perspective is easily recognizable 0r for sports photography.This is the Zuiko F4 which as such lenses go is quite light so you can use it handheld although support in the form of Monopods or Tripods really is best as the greater magnification increases the risk of camera shake.I bought mine in May 1997 and really like it's quality.