The hiring process in order was:
1- Online numerical test (8 mins, 80 qns)
Best tip I can give is don't even touch a pen or a rough paper for calculations, it is a complete waste of time in such tests.
2- Test meeting (on pen and paper at a venue)
(i) Numerical test (8 mins, 80 qns)
(ii) Sequences test (30 mins, 26 qns)
(iii) Numerical test (10 mins, 40 qns)
Information about these tests is given everywhere on the net. Only thing I would say is don't under-estimate the Sequences test. Reviews online make it look like all questions are of the form 2,5,8,11, ?. But only 4-5 questions were that straight forward. So just prepare well for this or you will be unpleasantly surprised. Also, DON'T use the rough paper.
3- Skype Interview (45-60 mins): (Motivational and Technical - Interviewer was a trader)
(i) Motivational questions: Tell me about yourself. Why Trader? Why Optiver? What does Optiver do? What is market making and how is it different?
(ii) Market knowledge: Do you follow financial markets? Current news in European markets? (Interview was for Amsterdam location)
(iii) Market Making game: Make a market on population of Sweden. Spread is limited to only 1 million. You will be given 500$ and 5 trades to make. You have to use full 500 euro by the end of 5 trades. Then the interviewer will make trades with you. Remember all the trades he is making and the profit/loss you are making as a result. The trades are binary so if he is buying at a better price than the expected price (expected population of Sweden), then you lose whatever you put on the line on that trade (out of the 500 euro).
At the end, he asked, what is the expected price in which you would have made most profit. So you should remember the trades made, the prices at which they were made, and how much you bet on each of them. Also, what is the expected price in which you would have broke even.
Just be fast on your toes to calculate these prices at the end.
(iv) Options knowledge: What is a straddle? When is it useful? What are the greeks you know of? What is the delta of a straddle? What is delta of an at-the-money call? Which model is used to price options and what are its inputs?
The questions started very simple but then became tougher requiring in-depth knowledge of the given topics. I would say if there is not much time for preparation, then prioritize these topics - Options basics, Trading strategies using options (combinations and spreads), Greeks, Black Scholes Merton Model. You can expect most questions to be from these.
4. There was one more Final F2F interview, but I did not clear the Skype interview.