OA:

1. https://leetcode.com/discuss/interview-question/1365052/amazon-oa
2. https://leetcode.com/discuss/interview-question/374846/Twitter-or-OA-2019-or-University-Career-Fair/337502
   Check for joshualian1989's solution in comments.(A perfect solution)

1st Round: (LLD)
Class diagram of Game Engine for Football Game.
Expected to write a code as well !!!

2nd Round: (DSPS)
LCA in n-ary tree.
Expected O(1) solution !!!

3rd Round: (System Design)
e-Bay design
Had discussion over full project lifecycle including testing automation frameworks and deployment frameworks.

Luck matters a lot.
But never loose hope, keep hustling and give your best.