Notice I said "before" and not "when." On the day we visited Toy Story Land in Hollywood Studios, the park's posted opening time was 9 a.m. We arrived at 8:40, were let right in the park, and ran to Slinky Dog Dash, where we were greeted with an already 75-minute wait time at 8:45. My husband balked, I jumped in line, and 20 minutes later, the line was at 2.5 hours, where it remained for the rest of the day.
If you're staying on property, transportation to the parks officially starts about 30 to 40 minutes before they open, but on super busy days, it usually starts earlier than that. Be on that first bus and you might just get in a couple of rides before the crowds create crazy wait times.