The first part of the job application procedure was 3 rounds of math questions. If you don't make it past the first round, it ends there. There are 80 multiple choice questions and you have 8 minutes. With fractions, the answers sometimes are not in the simplest form - if the answer is 1/3, it is possible that the answer 1/3 is not available, but 3/9 is. This makes it a bit harder to directly see the answers. A good way to train this part is by using the websites TraderMaths, arithmetic zetamac or rankyourbrain.
After the first round of math tests, the second round contains number sequences. You have 30 minutes for 26 sequences, becoming increasingly difficult. You have to get at least 20 correct to pass this round.
The third round of the math test contained calculations again. These were more difficult (at least different) than the first round, but you had more time. One question was for example: 2 hours, 35 minutes and 45 seconds - 1 hour 57 minutes 55 seconds = ?
After passing these rounds, I had a motivational phone interview. They asked me to elaborate on why Optiver, why trading, why it would fit me and why not. This was a short 30 minute interview and was not too difficult if you prepare this.
Next is the brainteaser interview. It was a challenging interview of an hour, where I got a couple puzzles that I had to solve (and had to estimate beforehand how long I would take). Furthermore they challenged me with a couple probability and expected value questions. I also played a market making game and he asked me 2 or 3 guesstimate questions.
Unfortunately I did not pass this round, but Optiver provided me with extensive feedback.
It was a very unique experience and I learned a lot from having this interview. Key is to prepare very well and to really prepare every round. I feel like I just did not prepare enough. Furthermore make sure that you enter the interview with a good night of sleep, you have to be really sharp to make a good impression.