took the amex hackerrank OA last week as part of their SWE hiring process. here's exactly what the format looked like.
format: 2 coding questions 90 minutes total HackerRank platform, standard editor, no IDE integration you can use any language (i used python) basic test cases visible, some hidden
difficulty: question 1 was easy. like, classic easy. array traversal, check a condition, return something. I finished in about 8 minutes and spent the rest of the time second-guessing myself. don't let it trick you, just solve it cleanly.
question 2 was medium. mine was string/sliding window adjacent. not the hardest version of that pattern but definitely required knowing the pattern. if you've done 40+ leetcode problems you've seen this type.
i did NOT see dynamic programming or graphs. other people I talked to got similar difficulty levels. the internet says some amex OAs have had harder problems but I think the new grad / entry-level track gets a gentler set.
timing: 90 minutes for 2 problems of this difficulty is very generous. i finished in 35 minutes and spent the rest on edge cases and cleaning up my code. don't rush.
what to prep: arrays, strings, hashmaps. that's probably 70% of it. two pointers and sliding window are worth reviewing. i wouldn't spend a ton of time on graphs/trees unless you're targeting a more senior role.
one thing: the amex OA doesn't have a debugging round or SQL section like some bank OAs do. purely algorithmic from what i saw.
results came back in about 6 days. good luck.