take it step by step.
First up, as said- KNOW the GOM. There IS a test; it IS hard, and people do fail.
After that’s complete, focus on the GOM material they’re teaching in class; and your aircraft systems. The oral will be a combination of those, dependent on the examiner (some focus more on GOM, some more so on aircraft systems/limitations)
Then focus on your EPCs and call outs for sim. KNOW what’s on the EPC checklist (so you’re not asking your partner to reference the EPC for something that isn’t there) and be familiar with the QRH.
The first day in the left seat, your muscle memory will be completely screwed up- don’t worry, by sim 2 it’s better and better.
Get through all that for the checkride.
Then move on to IOE.
People fail- where? I’m not sure, because you can’t just walk up to someone you know was awarded upgrade and is an FO and say “what happened?!” I’ve heard many struggle with single-engine sim stuff.. I’ve heard of a couple who couldn’t get past IOE and fed ride.
But, keep your wits about you and focus and you’ll be fine. It’s FAST. Staying on the same airplane you’ll go from day 1 of class to checkride in roughly 17 days, be on IOE 2 days later, signed off as early as the following week.