I am using OA for varroa control. It is effective if used correctly.
The key point is that the colony being treated must be broodless. The treatment of package of bees can be therefore effective.
In colonies which have brood, only a minor part of the mites are on the bees (called phoretic), the majority is sealed safely inside the cells, protected from the treatment.
When is the colony broodless?
1. A swarm or a package of bees.
2. Depending on the climate in winter there could be a period when brooding is suspended.
3. Artificial swarm.
In case of the artificial swarm, the colony is split in two, but not equally. One part has a queen and flying bees. This part has to be treated while there is no sealed brood. The best time is ~3 days after the split when the flying bees returned to this hive.
The second part of the hive has all the brood but no queen. Now it is useless to treat them with OA. You wait 3 weeks when all brood have emerged, but there is not yet new sealed brood. Now you can treat this part also.
It is often said that the treatment should not be repeated. This has to be interpreted so that a worker be should not bee treated twice. Since the workers are continuously replaced during summer, a colony can be treated theoretically after two months.
In practice one or two treatments per year is sufficient.