Online Assessments or otherwise referred to as "OAs" are interviews conducted to "screen" a potential applicant by testing their knowledge through a series of algorithmic programming problems.

These assessments are generally sent to potential applicants after they complete their job application for the company they are applying for.

Online Assessments often take place on a variety of different sites such as HackerRank and Karat. Companies often have a large pool of chosen questions to ask applicants that vary in difficulty and encompass various topics relating to certain algorithms and data structures. These questions are often limited by time, requiring applicants to complete the problem and pass any provided test cases within a certain time frame.

For example, you could be given this problem and need to solve it in 90 minutes: Given the root of a binary tree, invert the tree, and return its root.

Looking at the problem, you can pull out certain keywords like "invert" and "binary tree" and figure out it's a problem relating to the tree data structure. Furthermore, you can look at some tree/graph algorithms such as Depth-First Search and Breadth-First Search and see if they are applicable and how you would implement them for this problem. Think about certain constraints and edge cases and what kind of testing can you do to ensure your code passes common cases and doesn't get caught on certain edge cases.