Applied online, and a few weeks later received an invite to take the mental math assessment. I thought it was very easy, and finished all 80 questions with about 30 seconds to spare (I'm sure I missed a few though). I passed the assessment and then set up a behavioral phone screen with HR. While we were scheduling, I had an exploding deadline and asked Optiver if we could speed up the process. They said they could move the process faster and have an answer within a week. The behavioral phone screen was routine, and I was told I was moving on. Normally there are two more technical phone screens, but since I was expediting the process, I had both at the same time.
I thought the technical phone screens were fair, though I completely botched an easy probability problem. Unlike the other interviews I have done (and I interviewed at three other places), the person on the other end did not help me during the mistake or even point out that I was making one. (Later, I solved the problem and it was quite easy). He also emphasized me estimating answers, so turning fractions such as .59/.73 into decimals without a calculator. We then moved to an "estimation" problem, where he asked me to compute the volume of salt in the Atlantic Ocean. I think my thought process was reasonable and efficient, although I could have been horrendously off, who knows. Then we moved to quick estimation problems, where he valued speed over accuracy and wanted me to give answers in hundredths of a percent (so an answer of two would come out to 20,000). I estimated very quickly, definitely wrong on some of them. For example, give 100 divided by 370000 in ten-thousandths.
I received an email the next day saying I was not moving forward. I was a little surprised, because that same day I receive a full time offer at Jane Street (which is considered to be much harder to get), but I was unconcerned because Jane Street was my number one choice anyway.