I had only 30 minute online call with HR. Mostly behavioral questions and single technical question about programming languages. I did not get past this interview but recruiter was very kind and honest. I appreciate that MongoDB does not do any OA to get an interview like other companies and I was just invited to interview because of good resume.

Basic data structure runtime questions and Dsa quirks and then some code for 2 problems. It was a one-hour interview, where I had fully explain my thought process at each stage. They wanted to understand my what you were thinking and then they would let you code. Majority of the time was spent on coding and a little bit on runtime analysis. They care about correctness over anything else mostly, so for the first stage don't worry about optimality. Just focus on being correct and ask clarifying questions.