I have a bunch of oaks that came up in the yard last year and this year. Can anyone give me direction on how to prune them to encourage branching and trunk thickness?? I think these are Pin and Willow oaks.
"Can anyone give me direction on how to prune them to encourage branching and trunk thickness?? I think these are Pin and Willow oaks."
Unfortunately, your request is self-defeating. You can't do both of things and accomplish either. The best way to get thick trunks on oaks is to collect older material--your choice of willow and pin oak are great however, as both species can turn out to be great bonsai.
With these saplings, the best thing to do is plant them in the ground and let them grow--for five or more years unpruned, or until the trunks have attained the diameter (at about a foot above the ground) you're looking for, THEN cut them back hard--to a foot or so. You can also transfer them back into a container at the same time and continue the training process there. Several more trunk chops over an additional five years or so (probably more) will produce a nice tapered trunk.
Seedling oaks are a very long-term project, requiring time to produce a good bonsai base. Most collectors start with mature (or mature-looking) trunk at collection. Bigger specimens--up to 8 inches at the base--can be cut back to a foot high and branches regrown fairly easily. Both pin oak and willow oak have pretty shallow roots which make them a bit easier to dig than white oak or red oak.