I completed the OA and received an interview a couple of weeks after that.

I didn't have a referral or any other assistance.

Honestly, I was surprised I made it past the initial OA because, in my opinion, I didn't perform that well.

Focus on algorithms. They gave me Dijkstra's algorithm, which was pretty hard to complete in the approximately 25 minutes allotted. Make sure to talk through your entire thought process. I made that mistake, and the interviewer had to ask me what I was doing. I just was not prepared.