Oak Rough Bullet Gall wasps ( Disholcaspis quercusmamma) create stem galls on some species of white oaks. In Brooklyn, I find them on Swamp White Oaks (Quercus bicolor) with some frequency. Unusually for galls, these structures are also extrafloral nectaries, producing the sweet stuff that attracts ants, bees, flies, wasps, butteries, and other insects. (This thesis says 35 species of gall wasps, out of some 1300 Cynipidae species, have extrafloral attributes). It’s hypothesized that this helps protect the gall-inducers within from parasitoids and inquilines (which may or may not kill the host).
Backyard and Beyond 
I was inspired by this post to visit a swamp white oak near my Brooklyn home and post my gall observation on inaturalist.
Fantastic! White Oaks are also good trees to search for galls. In my Sunset Park neighborhood, there are a number of Swamp White Oak planted as a street trees, but they’ve been limbed up as they’ve gotten taller so the branches are now sadly out of reach.
https://www.gallformers.org is a good place to help ID galls even before you get to iNaturalist. This one allows you to search by host species.
Thanks!