I'm currently writing my Ph.D. dissertation, and I would like to use for my thesis exactly the same title as for one of my previous poster presentations.Is this OK?
I did exactly the same thing and am regretting it slightly. No one seemed to care, but for some reason the poster is listed on Google Scholar separately from my thesis but under an identical title. I could have avoided the "disambiguation needed" situation altogether by renaming one of them (preferably by saving the really nifty thesis title for the thesis, and using a work-in-progress version for the poster).
Use a title that makes sense for your dissertation. A dissertation is way more detailed than a poster so ideally your title would reflect this. Elaborating on the answer above, you might want to use another name, given that there is already a document with the same name.