Applied through their career portal. Got an invitation to Hackerrank challenge few days later. The challenge consisted of 8 questions (5 multiple choice and 3 coding questions) to be solved in 150 minutes. All questions were relatively easy except for the last questions which requires a lot of coding and designing an approach for the solution before digging deep into coding. After passing the challenge, I was invited for a phone interview with my HR recruiter and a Team Lead. They asked about my previous experiences and my motives for joining Optiver. They also reviewed my challenge solutions and asked followup questions on 1 of the problems. Next I was invited for an on-site interview round in Amsterdam. They paid for my flight as well as 2 night stay at a hotel near the company. The onsite interviews consisted of 1 coding interview, 1 system design interview, 2 HR interviews. The technical interviews were all e-trading related questions, however they assume zero knowledge in trading, so they do their best to explain all trading info relevant to the question. For the system design interview, you need to know the very basics of networking (TCP, UDP, uni/multi/broad-cast, how network latency and bandwidth works, etc.) in addition to general system design stuff. You are expected to discuss trade offs in your design. You are also expected to ask lots and lots of questions (about the requirements, constraints, trading stuff, etc.) Few hours after my interview, I got a call from my recruiter with the acceptance and the offer. Overall the experience was great and the offer was nice so I accepted it.