The 250mm f2.0 Zuiko. Maximum aperture is f2.0. This lens takes special 46mm rear filters. As far as I have been able to determine, Olympus never made any Neutral Density filters for this lens. The only other possibility is that you can mount a 128mm filter to the front of the lens, but the only commonly available option in this size is a filter that Pentax made for their medium format tele lenses and they aren't neutral density. The only other possibility for ND filters would be to modify an existing 46 mm ND filter. If you could find me a proper 2 or 3 stop ND filter for this lens, I will pay big bucks for it.
If I just wanted to stop down, why would I bother to use a lens like this? Why not just use a common 200mm f4 lens and be done with it? For bright conditions, I cannot shoot this lens wide open unless I use a slow film.