My colleague Nelson Lund says it is impossible to nominate "another O'Connor" to replace O'Connor:

First, even if one assumed the ideal replacement for Justice O'Connor would be another Justice O'Connor, the president could not possibly identify such an individual. Justice O'Connor's generally cautious and pragmatic approach to law means many of her significant votes depended on highly personal judgments about the likely practical effects of court decisions. Picking another pragmatist would actually guarantee we will not get the same judgments we got from Justice O'Connor. Every pragmatist is different.

He also says it would be unwise to try to nominate another O'Connor:

Second, Supreme Court candidates perceived as moderate, or even moderately conservative, almost always turn out to be leftists. History is filled with illustrations, from Harry Blackmun and John Paul Stevens to David Souter. If President Bush tries to find "another O'Connor," we will almost certainly end up with someone much more like "another Souter."