Adding an o to a word often creates a whole different word, which may or may not be a nonsensical one. But sometimes—like the great possum-vs.-opossum debate—it’s a little less straightforward. Recurring vs. reoccurring falls into the latter category (though this situation does also involve an extra c).
The Oxford English Dictionary’s definition for reoccur is “to occur again; to recur,” which isn’t helpful at all in any quest to discern a difference between the two terms—unless you’re really just seeking permission to use them as synonyms. In that case, go forth with the OED’s (and our) blessing.
For anyone interested in delving a bit deeper, however, there’s definitely more to this semantic story. Though reoccurring does mean something is occurring again, it doesn’t necessarily imply that the thing has already happened more than once in the past—or that it’ll continue happening in the future.
To put it in another way, the first time your dog ate your homework, it occurred. The second time, it reoccurred. If your dog eats your homework roughly once a week, on the other hand, that’s a recurring issue. It’s happened multiple times, it’ll likely happen again, and there’s some sort of regularity in the pattern of occurrence. Something doesn’t need to check all these boxes in order to be classified as recurring. Maybe your dog doesn’t stick to any sort of schedule when it comes to forbidden snacks, or maybe you learned to keep your assignments in a safer place and don’t anticipate further incidents. But in general, if something happens frequently, repetitively, and/or at regular intervals, you might want to describe it as recurring rather than reoccurring.
That said, people have been using reoccur interchangeably with recur (which dates back to the early 1500s) since it first showed up in writing in the 18th century. The OED’s earliest citation of reoccur comes from a 1734 English language edition of François Fénelon’s The Adventures of Telemachus, in which translator John Ozell notes that “the same objects frequently re-occur in this poem.” In other words, more than twice. So, again, you have our blessing to use recurring and reoccurring as synonyms.