AT&T TDP Software Engineer interview questions

Took OA and passed screening. Moving on to the technical interview. Any advice on what to study up on? I've heard OOP principles and leetcode easy are on there. So far the interview process has been really smooth and professional.

Pretty smooth interview process, they have four rounds. First round is the OA, which is a hackerrank with a few questions. Then is the screening interview with is around 3-4 behavioral questions and questions about location preferences. After that is the technical interview which is 2 leetcode questions, some debugging, and some systems design questions. After that is the final round interview with the associATe director, which is an hour long behavioral interview.