Generally, in the series like Op Center, Tom Clancy (with Steve Pieczenick, in Op Center's case) created the scenario for the books (setting, characters, perhaps some ideas for storyline), then unknown ghostwriters actually wrote the book.

The Op Center books I would refer to as "Clancy Lite" -- some of the same excitement as a Clancy book, but no meat behind it. The fact that Clancy himself didn't write it is probably why.