The model of truck was indeed a Western Star 5700 OP. A special variant of the 5700 XE. I have no idea how many were made and if it was available to buy, but it was an official variant.

The 5700 XE is actually not very well liked in real life. When it was added to American Truck Simulator a couple of years ago it went over like a lead balloon even with every part from the 5700 OP included that didn't require a license from Hasbro.

The should have stuck with the Peterbuilt 379 for Optimus, or gone with something like a Scania S730 V8 which was the most advanced truck at the time.