Oh, goodness. O'Brian has a lovely style, but you tend to get a few hundred pages into the story and realize that nothing's really happened, plotwise. Or at least I do anyway. But Jack is a wonderful character.
I think you might have hit the best part of Austen--her light touch. That she can make hundreds of thousands of words fly by, and, what's more, make every one of those words seem indespensible.
With O'Brian, of course, a little Austenian lightness goes a long, long way.
The rhythm of the way things happen in O'Brian varies a lot: many things happen off-stage and are narrated by characters in such a way as to bring out both the nature of the character and of the setting and of the different kinds of narrative that O'Brian provides. For example, at one point, Jack writes a letter to his wife describing a bloody knife fight in some small boats, but he carefully tones it down despite the fact that we the readers "witness" it all being replayed in Jack's mind. It's kind of like the Episode of
Band of Brothers where Captain Winters writes his report while reliving his single-handed bayonet charge against an SS battalion...though (perhaps paradoxically) the cinematic narrative tends to distance us from Winters while Jack's attempts to avoid verbal bloodshed gives us the illusion of an incredibly intimate glimpse of his personality.
In Fay Weldon's screenplay for the 1979 BBC P&P (by far the best cinematic version of the book) a similar double play with reading and writing gives a great view of how Elizabeth operates...indeed how she changes her mind and says "I never knew myself until now"....ie when she reads Darcy's letter.