Online Assessment (OA) refers to the coding test which companies take to shortlist candidates for interviews.
It generally consists of 3-4 coding problems which we have to solve within a limited time (generally 1-2 hrs).
People who don't give contest on codeforces and rely on leetcode for their preparation often struggle to clear this test.
The best thing you can do is practice past OA problems which is available on thejoboverflow.com. Practice as many medium and hard tag problems from this site as possible.
Along with it, give every leetcode weekly and biweekly contest. You can also give virtual contests.
Click Here to know, how to use leetcode efficiently.