If your Operational Skill power has fewer dice than your stat + skill pool to use it, your stat + skill pool is reduced to the same number of dice as the power. You need 10d in the power *and* the pool (or bonus dice from Spray or Augment) to roll a full 10d when using the focus.
For example, someone with Coordination 3d + Weapon(Big Clunky Machine Gun) 5d (for 8d total) and a Big Clunky Machine Gun focus with Spray +3 but only 4d in the focus, will roll:
3d + wd (skill reduced by Operational Skill, keeping the wd) + 3d (spray) = 6d + wd
One could argue that Operational Skill is a penalty and therefore eats hard dice before normal dice, but I don't think I'd read it that way unless Shane comes along and says so.
-- Daniel