okay so I just did the UHG/Optum online assessment last week and wanted to share what the format actually looked like because I was searching forever and couldn't find anything specific.
they send a HackerRank link. you get 90 minutes. in my case it was 2 coding problems.
problem 1: felt like a medium on leetcode. it was basically a graph traversal, finding the shortest path with some constraints. nothing exotic but you do need to know BFS/DFS cold. i used BFS and it passed.
problem 2: a string manipulation / parsing problem. given a structured string with some health-data-ish format (not real HIPAA data obviously, fake patient IDs and codes), parse it and return some aggregated result. easier than problem 1 honestly but easy to mess up edge cases around empty fields.
the difficulty felt like a step below what I'd expect from Google or Amazon. if you've been grinding medium leetcode problems you should be fine. i don't think i saw anything that would be hard.
things i'd do differently: read the problem constraints carefully. problem 1 had a note about the graph being disconnected in some cases and I almost missed it. also test your code with edge cases before submitting, there's no partial credit if all test cases fail.
there was a 5-minute window after submitting where you could review (read-only). no looking up anything obviously.
no system design in the OA, that comes later in the panel. this is just the filter round.
i passed and moved to the panel interview, which was scheduled about 12 days later. the wait was the hardest part.