Both a 0W-30 and 5W-30 are what is known as multi-viscosity motor oils. They are often confusing to people however, in that they are both SAE 30 motor oils at your engine's operating temperature. The 0w and 5w, or for that matter, 10w and 15w, referr to the oil's performance from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests. Basicly how easy the oil will allow your engine to turn over when it is cold.
However, again at operating temperature, both the 0w-30 and the 5w-30 are SAE 30 motor oils. So no, the 0w-30 is not thinner than the 5w-30.