From what I can see in the images it doesn't look that bad, but it is hard to see details with the images provided. A lot of the brown I see are the undersides of the leaves, which is common, as they typically have a brown fuzz on the lower leaf surface. Yellowing, browing and splotchiness of older leaves is also normal as the tree naturally sheds older leaves each year. Typically these are at least two year old or older leaves. If the new growth is yellowing and showing some stress then that is something that needs to be explored more.

Over the past several years we have had some extreme weather events that have affected many tree species including southern magnolias, so some of the thinning could be a result of the stresses brought on by these weather events. 

I do see some very large limbs/stubs that have apparently been pruned improperly and should probably be cleaned up. For assistance on proper pruning see our fact sheet Pruning Ornamental Trees, Shrubs, and Vines or consult with a certified arborist. 

Since this sounds like a newer property for you, you might consider doing soil tests if you haven't had one done in a while to determine if soil nutrient levels are appropriate for the tree and surrounding landscape. 

For local assistance you can also reach out to the Oklahoma County OSU Extension Center https://extension.okstate.edu/county/oklahoma/, they can also help with a soil test if you choose to do one.