BYO has a decent article about using/steeping oats here
(the second question, 1/2 way down the page). It depends on how much oat flavor you want, but you can use up to 30% of it in your grain bill. You may be better off doing a partial mash if you are making an extract recipe or even a BIAB mash, however.
Many people roast there oats before using them which adds flavor and color, but this usually requires "curing" them in a paper bag for a couple of weeks which may not work for your time table. There are several posts around HBT regarding roasting oats if you want to know more.