Different players altogether. Bridcutt is energetic and aggressive playing across the park in front of the back 4 - O'Sullivan is more creative playing much further up field.
If you're saying that O'Sullivan shouldn't have been put out on loan and kept with the first team squad then I agree. Slade has said he dwells the ball too long - that shows how inept a manager the guy really is.
Crap Sunderland team or not, the question is whether Bridcutt is a better option and would bring more to the side than Dikgacoi or Gunnar. On pace alone the answer is a huge yes. It's all very well putting your foot in, but if you're as slow off the mark and over the ground as those two, a quick opposition midielder can leave them for dead.