Cardio can require as much or more protein in the diet as lifting. While it doesn't build much muscle, especially hard or long cardio can require a good deal of muscle repair.
Generally, with both cardio and lifting you want to get some simple sugars (not just protein) in quickly to promoted an insulin spike. Add some maltodextrin or even regular granulated sugar to your protein shake. This helps with getting glycogen stores replenished, but also helps transport the protein into the cells where it can begin going to work.
The carbs in oats, esp. w/ the fiber takes a lot longer to raise your blood sugar. Still better than nothing, but I'd shuffle them around to get fast digesting carbs in right away, cut the oatmeal and have a more regular meal about an hour or two after your workout (make sure you are still getting at least 30g. of fiber a day in your diet).
If you are wanting to lose a little fat even while adding the extra food, I'd just make sure that now that your cardio and lifting are on different days that you make sure both are as high quality as you can.
You can also add five minutes of slow cardio to the end of each workout to burn some extra calories without adding extra stress- sort of a pre-cool-down. You will also have the post-exercise metabolism boost twice as often as you did when you combined your cardio and lifting. It wont be twice as many calories, but being able to get high quality in on both sessions should produce a boost in calories burnt compared to what you were doing.
In other words, you should have much more productive cardio sessions (presuming before cardio followed lifting, if not it will be more productive lifting sessions) than you did and offset the additional calories of the extra post-workout food.
Just keep an eye on things, and if your waistline starts heading in the wrong direction, adjust by offsetting some calories elsewhere in the diet or doing a bit more exercise. I think you'll find you are pleased by the results.