UPDATE III: University of Illinois Professor Robert McCheseny made the following findings -- using a Factiva search -- that were sent to me via email:
Olbermann had criticized O'Reilly 40 times on air from February-May 2009, and had made O'Reilly one of his "worst persons in the world" 23 times. In June and July O'Reilly received only one negative mention, in early June, and never made the worst person list.
Moreover, Olbermann criticized Rupert Murdoch 25 times on his program between February and May 2009, and only once in June and July.
Conversely, O'Reilly went from making 27 negative mentions of General Electric in February-May 2009, to just two in June and none in July.
I suppose if one is really desirous to cling to the notion that Olbermann wasn't muzzled by GE as part of its deal with News Corp., one can find a way to do that, but doing so would take extreme amounts of mental effort.