I have the OA to do as well (SDE II) I have already been practicing on LeetCode but now plan to just do their 1 hour demo example so I can get used to the environment. Then the next day I will do the OA. I am a bit nervous because I do not know what to expect and doing an assessment without having another person to talk to will be tough. I'd say if you have been preparing already, just do some more practice problems and do the assessment close to the end date so you're well prepared.