While the offensive systems are vastly different, O'brien still utilizes a lot of the ZBS that Kubiak did. RAC's system isn't so much different than Wades either. Just less interior pressure allowing for LBers to do their work and a lot more man coverage scheme. The roster turn over is due to Bill's vastly different player philosophy to Kubiaks/Wades. Wanting a much bigger, stronger, smarter, athletic team that can impose their physical will on opponents. Unlike Kubiak's/Wade's more finesse style. http://espn.go.com/blog/nflnation/post/_/id/171039/how-the-texans-reshaped-their-roster-to-bill-obriens-specifications