Wright thumped O'Brien down the ground for the solitary six of the innings from the final ball of the 16th over but fell two balls later as Rankin returned and the Sussex all-rounder could only spoon off the toeend of the bat to extra cover.
As the action swung to the other end, Scott Sinclair toe-poked a chance weakly and Clinton Morrison also failed to get power into an effort after a sweeping move.