Massive Clinton landslide due to Wilson's pro-abortion position. This causes many conservatives to stay home/vote third party. Also, being warned about Pearl Harbor in advance didn't hurt FDR, and being warned about 9/11 in advance didn't hurt Bush Jr. Thus, I doubt being warned about Oklahoma City in advance would hurt Clinton, since he could always blame someone else. Finally, Bush Jr.'s inexperience causes many people to question Wilson's judgment in choosing him as VP and vote for Clinton (similar to McCain/Palin in 2008).
I don't think it would be that much of a landslide but Clinton would've won by alot. The VP isn't even a top 25 issue and Bush never had advanced knowledge of 9/11 that was credible prior to the attacks.
http://en.wikipedia.org/wiki/Bin_Laden_Determined_to_Strike_in_United_States
Most of the intelligence community recognized in the summer of 2001 that the number and severity of threat reports were unprecedented. Many officials told us that they knew something terrible was planned, and they were desperate to stop it. Despite their large number, the threats received contained few specifics regarding time, place, method, or target. Most suggested that attacks were planned against targets overseas; others indicated threats against unspecified "U.S. interests." We cannot say for certain whether these reports, as dramatic as they were, related to the 9/11 attacks.
Had we had better funding for the CIA and FBI throughout the 90's, it could have been prevented. This gave us a month to stop such things. I'd like to see someone capable of figuring that out.