There's no ramp on taxation after 40 hours in any jurisdiction I'm aware of-- just a ramp on income tax based on amount earned.
If you're making $800/week ($20/hour) and seeing withholding at 10%, the marginal withholding might be 20% on a step up to $950; on the other hand, you're getting time-and-a-half, so you still go from taking home $18/hour to $24/hour.
(And, of course, for occasional overtime work, you're going to get a lot of the additional tax withholding back, since your uneven income causes you to be overwithheld).
