This is a concern, as several diseases can affect oaks.  If they haven't leafed out this year, they may well be dead or dying.  What variety of oak are they? Red oaks are more susceptible to oak wilt, although it can affect other varieties.  The two-lined chestnut borer has killed many oak trees in Minnesota this year.  The trees were harmed by our three-year drought, which made them susceptible to the beetle.  I'm attaching a couple of links about oak trees.  Read the information and look at the photos to help you determine what is going on with your oaks.  If you have several trees, calling in an arborist may be a good idea.

https://www.missouribotanicalgarden.org/Portals/0/Gardening/Gardening%20Help/Visual%20Guides/Oak%20Problems.pdf
https://www.mda.state.mn.us/plants/plantdiseases/suddenoakdeath
https://www.treesaregood.org/findanarborist

Attached are a couple of photos that I should have sent the first time. Both of these trees appear to be dead, and they were healthy last year. The second one has some new growth coming out of the trunk, but they don't look like oak leaves. there are other trees nearby that look healthy. 

Is this something that needs to be addressed right away or would it be better to wait till fall/winter to take them out?

Don't remove these trees until at least November through March.  Even dead oak trees can attract the beetle that causes oak wilt and may result in more infected trees. If you have other oak trees, it would be worthwhile to consult an arborist to determine the exact cause of your trees' decline.  By doing that, you may be able to forestall deterioration of any other oak trees.