I'd say the plunge feed rate was to much, if your going to plunge with a tool designed for side cutting, its best to ramp into the cut if you can or use a drill operation first to open it up for the cutter to fit inside and then start side cutting. Or you can use a spiral ramp cut, did you use either of those or just plunge straight down into the oak?

Edit:
Is the second pic where the cutter broke in the oak?, it looks like it is and suggests a straight plunge...