Ask Mr. Smarty Plants

This is kind of like a word association game: you say "black", I say "white"; you say "night", I say "day"; you say "trim oak trees", I say "oak wilt". This is because oak wilt is one of the most destructive tree diseases in the United States and is killing oak trees in Central Texas in epidemic proportions. It is an infectious disease caused by the fungus Ceratocyctis fagacearum which invades and disables the water-conducting system in susceptible trees. Oakwilt is spread by tiny wood boring beetles called nitidulid beetles that are attracted to the sap of oak trees. Fresh wounds in oaks draw these beetles like honey draws flies. The adult beetles carry spores of the infecting fungus on their bodies as they go from tree to tree and thus spread the disease.

Does this mean you can never trim you oak trees? No, but as in a lot of things, timing is everything. And, all species of oaks are not equaly susceptible to oak wilt. The nitidulid beeltes are least active in very cold weather and very hot weather, ie. in mid-winter and mid-summer (like right now). 

Mr. Smarty Plants suggests reading the material from the Texas Oak Wilt Information Partnership and the Texas Forest Service to become more knowledgable about oak wilt and the care of your oak trees. This article from  the USDA  is very extensive, and tells you more about oak wilt than you probably want to know. It is well-illustrated.

The Texas Forest Service also has an online article How to Prune Trees that should be helpful.