You mention taking the course—is that the complete (live) course? Interact? Self-Study Toolkit? Just trying to figure out which materials you have access to.
If you want to review anything before taking a practice test, I wouldn't recommend doing new OG problems (beyond the review sets assigned in your syllabus, if any—depends on which course you're taking). Save the ones you haven't done yet for after you take your next CAT, so that you can use the Random Set feature in Navigator to make random sets.
Rather, make your own problem sets out of OG problems that you've already tried in the past. They're the best to check your skills before you go and take a test. You'll remember some, and then you can just use those to verify that you do still know how to do all of the needed steps to get to the right answer in an efficient way. You won't remember others—and so those will essentially be "like new" for you and good problems to try again and review.
But I wouldn't do a huge amount of review before you go to take the practice test. The practice test itself is your best opportunity to figure out your current skills / strengths / weaknesses. You'll use the results to plan out the next several weeks of your studies (before you take another practice test and repeat the whole process). So really think of the practice test as your diagnostic tool to figure out what you've learned well and what still needs more work.
In general, once you're done with the course (or whatever your initial study phase was), you've been through all of the material once, so your further studies will be driven entirely by your analysis of your current strengths and weaknesses. This is the cycle:
– Take a practice test under 100% official conditions.
– Analyze the test. Use our CAT Tracker analysis spreadsheet (found in Atlas / your syllabus) to analyze your results and then use that analysis to figure out what you want to prioritize in your studies between now and your next practice test.
– Don't prioritize your biggest weaknesses. Just put those on your "bail fast" list for the next practice test. (This is within reason. You can't put all of algebra on your bail fast list or all of Critical Reasoning on your bail fast list. But you can put quite a lot of individual content areas on your list.)
– Do prioritize your "opportunity" weaknesses. You got it right but took too long. You got it wrong but made a careless mistake. You got it wrong but you understand it now and feel comfortable learning how to do it.
– Spend about 2-3 weeks addressing whatever you decide to prioritize. Do this mostly by continuing to do practice problems under timed conditions (though, of course, also review any specific rules/facts/content that you need to study and drill any individual skills that you need to improve).
– Then take another practice test and repeat the process. (And, on that test, also practice guessing fast on your "bail fast" problems!) Each time you take a test, you'll update your "current strengths and weaknesses" list and your "bail fast" list, based on how that most recent test went.
If you do want to try an OG problem from a specific area but you don't have any more "already done" problems to re-try, it's ok to pick out a couple from the OG yourself and try them. In any course / syllabus that provides you with the OG, the syllabus also contains a list of the OG problems organized by content area, so you can use that to find specific problems. Although, if you really want to test yourself, I recommend first scanning the book yourself to see whether you can recognize a problem of the type that you want. (And then, if you're not sure whether you found one in the right category, you can look it up on the OG problem list in the syllabus to see whether it really is the category that you wanted.)
Good luck!