I've been grafting oaks for over 25 years. Got hooked up with NAFEx and NNGA folks selecting low-tannin oaks as potential nut crops. Have several of those low-tannin (white oak species) selections as well as a number of hybrids and large-acorn strain bur oaks.
They're no more difficult to do than apples, easier than persimmons. Guy Sternberg is a friend, but I'll disagree with him on the difficulty aspect. If you can graft persimmons, I guarantee you'll be successful with oaks.
Timing is maybe a little more critical... best time to put on dormant scions is just as the rootstock is beginning to unfurl leaves. I've grafted some later in the season, but best success is just as rootstocks are leafing out.
White oak species go on white oak species, red/black on red/black (see further info below). I'm not up to speed on the Eurasian species... but suspect that stuff like Sawtooth will work best on other members of the Cerris subgroup.
Across the white oak group, there appears to be no issue with incompatibilities, though I'd probably shy away from post oak as an understock, as it's so slow-growing. I use mostly bur oak seedlings for rootstocks, as that's what I have most readily available. With the white oaks, if you choose a rootstock that will perform in a particular site (soil type/pH, moisture level, etc.), you can graft on species that wouldn't necessarily do well in that site. I'm not saying you could have success with a zone 9 live oak in a zone 5 site, but you could, for instance, put chinkapin oak, which likes upland sites, on an overcup or swamp white oak understock and grow it in a lowland, almost swampy, setting.
Red/black oaks are a bit more problematic. There are peroxidase enzyme incompatibilities that may result in graft failure/rejection years down the road. I have grafts of Q.nuttalli 'Macon' on Q.rubra, Q.falcata, and Q.palustris understocks, grafted in 2014... all are growing well, the grafts on Red & Southern red look fine, but the graft union on Pin oak looks wonky, and I suspect it'll fail at some point.
A simple bark graft works just fine; I cut off the rootstock, make a vertical incision in the bark, lift the edges. Make a long sloping cut in the base of the scion as if you were going to make a simple whip/splice graft, then gently 'shave' just a little off the two lateral edges to expose cambium on the sides, and slide it down into the vertical incision, stopping when the cambium layer at the top of the cut is just above the top of the beheaded rootstock. I wrap graft union and scion(in its entirety) with Parafilm M, then overwrap the graft union with a rubber band - or, more recently, a layer of masking tape and then rubber band. Rubber bands will photodegrade... I never need to cut them off.
If you have even a modicum of experience grafting, you should expect success rates above 80%.
