Hi Karen,

Unfortunately, there's not a selective herbicide that only works on thistles but not on oaks.  The key for successful herbicide use is going to be choosing products that do not have soil activity (to protect the oak roots) and applying them carefully so that they touch only the unwanted weeds and not the oaks.  

Carefully read the label instructions for any product you are considering; they will all include stipulations/warnings about applying within the root zone of desirable trees and shrubs if applicable.

Managing the musk thistle mechanically (that is, by pulling or cutting below the root crown) may be the best option, since it can be eliminated this way.

For the Canada thistle, though, an herbicide is probably warranted, though pulling consistently and persistently can help too, over years.  One herbicide to consider is Glyphosate, since it has limited soil activity.  It will take more than one treatment to eliminate the thistle, but is lower risk for your desirable plants.

Again, just be sure to read and follow the label instructions of whatever product you choose.

I hope this is helpful.

Cordially,

John