Ah, this takes me back. I applied back in 2014 when I had just turned 16. The interview was really fine, he asked me why I wanted to work there, what skills I have, I believe he asked for a weakness too. He also asked me what 3 words my friends would use to describe me. As long as you're confident and don't come across as a complete and utter idiot you'll be fine. Don't worry about taking time to think, or stuttering, or not being sure. It's more about how you come across, be yourself! Be charismatic. Before your interview think of a few qualities you have, think of a time where you've been a good friend or helped a stranger.
As for the OJE, he never actually asked me to do one. After my interview I asked if I was going to do an OJE and he just sort of said "Oh... Yeah, hang on..." and gave me a cloth and asked me to wipe a few tables. It was really odd.
The best advice I can give you, as cliché as it is, is DON'T BE NERVOUS! There aren't going to be any curve ball questions in there. I can't tell you much about the OJE, but don't worry if you mess up, because when you start working there you'll be messing up all the time, trust me. 2 years on and I still make loads of mistakes every day! For example today, a guy said "I gave you £10, didn't I?", I couldn't remember so the manager had to count the till to figure out whether I owed the guy money. It's no biggie, stuff like that happens all the time.
I'll say it again, DON'T WORRY and good luck!