Optiver doen't interview for a specific team, but rather first decides whether the company wants you and then places to to a specific position. The first stage was a 48 hr challenge. The questions were all pretty basic, except for the last one, which took me a long time. After that, there's a 30 min HR interview. Then a 45 min technical interview, a lot of which is a talk about the challenge. After all that, I flew to Amsterdam for an on-site interview (Optiver covers the flight and hotel expenses). This included two technical interviews, one HR, and one lunch with peers. The whole process was very positive. The recruiter was always available for any questions and all the interviewers at all stages were very nice. I had to give up the very nice offer they gave me, because of personal reasons, but I think it could be a great company to work for.