oat flour is oats milled down to a flour. Its a great way to bulk up. You can put some in your protein shake but its even better to let it soak in water and make oat milk, then strain out the larger pieces so you have an easy to drink concentrated carb drink that is full of vitamins and other good stuff. Some people bulk with shit carbs or buy those protein powders with added carbs. I recommend oat milk over anything. You should never consume carbs without vitamins, there is a reason why they add vitamin c and a few Bs to energy drinks. Oats arnt too bad for protein too, i think they are 15% protein. I use bananas for flavour coz of their high potassium.
Here's is a few more thoughts:
Increase your salt and other electrolytes & magnesium. All through my 20s I endured chronic leg cramps because I didnt consume enough electrolytes.
I recommend a book by Brad Schoenfeld called max muscle plan. Its got a great progression program with photos and descriptions of all the exercises in the plan. I wish I had this book when I was your age. It switches between the strength training I describe below and Brad's hypertrophy training.
I recommend "5X5 strength training". Its very popular so you might find its hard to find the original program by Bill Starr. I have Bill Starr's book, I dont recommend it because I can tell you the program here: Its the big heavy barbell lifts; dead lift, squat, bench, press, row. Do some warmups then hit 5 sets of 5 reps of every bar exercise with progressive weight increases, 3X per week. A lot of people miss that 5X5 was popularized by a football coach and so has proven to work as a compliment to playing sports, not as a complete system on its own.
Do hip thrusts as a warm up before dead lift and squats. I was given hip thrusts from my physio therapist and it did heaps for me. Look up jeff nippard on youtube, he does an interview with an expert about it. It activates your muscles better than doing some practice dead/squat sets.
Get a golf ball and a foam roller. The stretching room at the gym is only good for perving at ladies. Foam rolling > lady stretching. Stretching does not release problematic & overworked muscles. The golf ball is for the very deep trigger points. There are a few yoga exercises worth doing as soon as you get out of bed. You can look them up on youtube. I like the indian push up and a hamstring stretch that I actually dont know the name of. That should be all the stretching you need.