There is 1-2 round phone screening followed by one onsite interview. The phone screening is coding where an engineer ask you coding questions, there are system design questions during onsite interview. You can refer to cracking the coding interview for preparations.

Sending this is a bit unfortunate because my first interviewer was a great experience. However, the bar was dropped severely with the second interviewer. Interviewer provided a simple question in perhaps the most unintuitive explanation. I spent 15 minutes of the interview trying to understand what was even wanted in the question. Seems very much like he just expected me to have the question memorized beforehand. Furthermore, he insisted he "wasn't working" but repeated assumptions I had already made to him while going over my solution. Definitely not a bad experience with the interviewer personally, but, if any interviewers at OfferUp take notes from here, take 10 minutes to simply understand how to state a question quickly and clearly so the person you are interviewing can do their best.