Agree with Josh.
Also, the straight answer to your question is "don't know".
Reason being, we don't have information how long the oxalic strip has been in there, or if it has worked, and what the current mite levels are in your hive.
It may at this time be asking too much of a brand new beekeeper to accurately answer those things, so that is why I'm agreeing with Josh. IE, the sure way to know your bees are free of varroa mites would be to remove the oxalic strip (because they can give mixed results), and put in a conventional treatment.
The best recommended conventional treatment (in my view) would be Apivar (not to be confused with Api Life Var). The reason I would recommend Apivar is because it is pretty much idiot proof. You put it in the hive, take it out 8 to 10 weeks later, and job done. Drawback is you have to buy a pack of 10. But if you give your location there may well be someone with some spare strips so you don't have to get a full pack.