Alex,
In general, the term "tube lens" refers to a secondary lens that is used behind the newer "infinity" type objectives to complete the task of image formation. The objective essentially turns a small subject close up into a virtual image at infinity, and then the tube lens focuses the virtual image from infinity to form a real image on the camera sensor or at the microscope's eyepiece. See our
FAQ: How can I hook a microscope objective to my camera? for more discussion.
When Charles says "Nikon tube lens", I believe he means the specific tube lens made by Nikon for use with their infinity objectives. That tube lens can be purchased as a separate item, most conveniently as an Accessory to the
Nikon objectives sold by Edmund Optics. There's nothing magic about the Nikon tube lens; in particular it does not perform any special corrections for chromatic aberration. An ordinary telephoto can be used instead, as described in the FAQ. The significance of Nikon tube lens in this thread is that it indicates the Olympus objective gives good results with a tube lens that does not have special corrections. I take this to mean that it would work well with an ordinary telephoto also, as in the FAQ.
--Rik