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Stefan,
An OAG eliminates differential flexure. It is not often that anyone could image at moderate to long focal lengths and use a guide scope without some differential flexure. An OAG is the way past all problems. The first time I setup an OAG it was difficult but the main thing to remember is that you need to have the guide camera a similar length from the pick off prism to the sensor on the main camera from the pick off prism. So focus the main camera first then focus the guide camera. It does not need to be perfect focus, slightly off is ok. Guide settings will vary too because you will be guiding at the same focal length as the imaging scope. CCDware have a great tool that helps you determine what your guide settings should be for your system. It's worth chasing it up. Try aggression about 8/10 first and then back it off or up it from there.
I would never go back to a guide scope. I have used OAG with an TSA, FSQ, with an RC and now with a Newtonian. Persistence pays off.
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