Oxalic treatments are useful, but I would not use them specifically to treat a hive that has a high mite load. This year I did, however, follow the lead of some local commercial beekeepers, and use oxalic acid strips between the spring and autumn synthetic treatments to keep mite numbers lower, and then I put oxalic strips in again after I had treated with Bayvarol in autumn (and tested for mite numbers) to give the girls a bit of extra help through late autumn and winter. There is always a risk that after synthetic autumn treatment (and your hives test fine) they can get reinfested from someone else's bees - hives somewhere else that were not treated, or had failed treatments.
I did this treatment plan (as described) this season, and all five of my hives are looking good for winter. One of my hives even has four frames of capped brood, both sides, and the girls are still bringing in little sacs of pollen.
I made my own strips, wearing chemical resistant gloves, a waterproof jacket, safety goggles and respirator, because I think safety should always be paramount. I used a stockpot I bought from an op shop for the mixing, a cooking thermometer to get the right temperature, and used the cooking pot hob on one end of our outdoor barbecue, so it was all done outside in fresh air. I put my first ever strips into one test hive first, a week ahead of the rest, so I could be sure I had the formula right and the strips were not detrimental to the health of my bees.
I developed this treatment plan because my very first year of beekeeping I lost both my hives in late autumn/early winter. In retrospect I had treated too late for varroa. I saw both colonies full of bees, but did not realise that as the colonies reduced for winter the mites increased dramatically, and the viruses took hold, and within weeks I lost both hives. When I worked it out I felt so bad I nearly gave up beekeeping. I later did the Level 3 course at Polytech and learned so much, and this year as I wintered down my hives I knew what to do, what I was seeing in each hive, and how to get my girls through Winter with healthy bees, and food for Spring.