I also have an Olympus, and was looking at an extra lens to add to the one that came in my kit. Being new to photography, I'm not sure what would qualify a telephoto lens over a large zoom lens (is there a difference).
Either way, when I was looking at some of the (rather limited) options available to users of the 4/3 system, I saw that you could easily get the Olympus Zuiko™ 40-150 mm f/3.5-4.5 Lens for under £200 new, and for closer to £100 on a popular auction website.
I also went onto the Sigma website and noticed they manufacture 3 lenses for the 4/3 system - one of them being the Sigma 55-200mm f/4-5.6 DC which is available for £99.99 on a popular high-street photography retailers website. Unsure on the details of that one , does anyone know if that the AF will work?