I only made it to stage three of the interview process. Stage one was an online cognitive test. It was rather typical of this kind of test and not too difficult. Stage two was a single HackerRank coding problem that focuses on algorithm instead of something like object-oriented design. Only C++, C# and Java (maybe also Kotlin?) was allowed. I didn't do well, since I wasn't familiar with Java, and I had not practiced algorithm before hand. Somehow, and with much appreciation, I was contacted for stage three - an online interview. I don't know what got into my mind at the time, but I gave many responses which I later realised basically said to the interviewer that I was not the person Optiver was looking for. Stupid of me, and not surprisingly, I failed at this stage. Overall, I'd say the interview difficulty was slightly above average, which was probably fair for an intern position. It was my own fault that I didn't prepare myself better failed it.