Went through Epic's online assessment last month. Since there's a lot of confusion online about whether it's LeetCode-style or something else, here's what I actually saw.
Short answer: it's closer to a general coding competency test than a grind-the-algorithms-until-you-bleed LeetCode session. Which honestly was a relief.
Format: I got two coding problems and one written question. Timed at around 90 minutes total but I finished in about 65. You code in a browser-based editor. You can pick your language; I used Python.
Problem 1: Working with a list of records, filtering and transforming. Something you'd actually do on the job. No exotic data structures needed. Brute force would work but a cleaner linear pass is obviously better.
Problem 2: String manipulation. Count something, find a pattern. Easy-medium range. If you've done any LeetCode in the last month you'll be fine.
Written prompt: "Tell us about a time you had to troubleshoot something complex." They want a paragraph or two. Don't overthink it. Just write clearly.
What I noticed compared to other OA formats: no trick questions, no dynamic programming, no graph traversal nightmare. They seem more interested in whether you can write clean, readable code than whether you've memorized the exact Dijkstra's implementation.
That said, the onsite apparently has a problem-solving round that goes a bit deeper into CS fundamentals -- runtime analysis, OOP concepts, some recursion. So don't skip the basics entirely.
My prep: about two weeks, mostly LeetCode easy/medium, some array + string problems, a quick review of Big O notation. That felt right.