I live out near there, but I never use public transport so I really can't say. I do know that the Metrolink line that runs near Ontario is the Riverside Line and it only runs Monday-Friday.
Here are the web sites for the bus and Metrolink services:
http://www.mta.net
http://www.metrolinktrains.com
And also the Ontario Airport web site which might give you some transport info:
http://www.lawa.org/ont/ontframe.html