certified used car leaking oil after one week, <?

You should expect them to fix the leak free of charge. If it's certified used, it was supposed to pass a multi-point inspection and that inspection should have included finding and correcting any fluid leaks.

My guess is that the problem is something simple like either the oil drain plug didn't get tightened properly or the oil filter isn't tight (a loose oil filter can cause a large amount of oil to leak out in a short period of time). I doubt there's a major seal or gasket leak or they would have (or at least should have) caught it and fixed it when inspecting the truck during their certification process. If it were me I'd call the dealership, explain the problem, and insist that they pay for the towing service since I (you) just got the car. The worst they can say is 'no' to the tow cost, but either way I'd tow it there if it's leaking a lot of oil. No need to risk driving it and damaging other things that won't necessarily be covered if you run it low on oil (or if the oil all suddenly leaks out). Good luck

Call them a tell them what is going on. They should offer to tow it in. The smell is likely oil dripping on something hot. Expect this taken care of with apologies from them.

You should expect them to listen to your problems and then do something about them.

How you tell them about your problems may have a big effect on how they deal with them. Speak to them civilly and calmly and expect them to respond in the same manner.