MOT Power supply issues
I'm sure you figured it out by now, but you need to limit current to your power supply by attaching two reactors (modified MOT's) as inductive ballast. The arcing you experienced was most likely in the outer two MOT's as they are not build to handle more than 3000 V. Oil submersion seems to be necessary at least for the outer two transformers. Go to Steffan's Tesla Coil Page. There I have instructions on how to hook up the reactors. I have been testing without oil submersion and haven't had any problems so far.