Does coating the dough with olive oil keep tomato sauce from absorbing into it?
In general, it would seem that coating a dough in olive oil before topping it would keep things from getting into / staying onto it. I think your best bet though is to just try it out, maybe with just a little piece. I imagine if you really want the sauce to get into the dough you should put oil on afterwards, but it might still work. Give it a shot!
Don't know which dough recipe you're using but the one I use has some olive oil in the dough and I haven't experienced the problem you describe. Your problem may be that your sauce isn't thick enough. The sauce should be thick. Don't let the pizza sit around after you apply the sauce and toppings. Get it into a hot oven asap. If the sauce contains too much liquid then that may be causing your problem, with the dough absorbing some of the liquid from the sauce and becoming soggy or uncooked. Also, you need a hot oven and preferably a preheated stone to cook your pizza on. If your oven isn't hot enough the dough can absorb liquid from the sauce and become gooey. I do pizza frequently and the only olive oil I add to the dough after mixing is for brushing the outer exposed crust with olive oil, prior to putting it into the oven, so it will brown nicely.
Olive oil does keep the sauce from absorbing a little. Like holds99 said, if it sits around on the dough for too long it can absorb some. I brush a little olive oil and lightly sauce my pizza and my sauce is thin, basically pureed canned tomatoes. That's just the way I like it. If you like a saucier pizza it might absorb a little, but quickly into a hot oven with a stone should take care of any problems. You really need a hot oven with a stone to make good pizza.