Chickens are omnivores with complex nutritional needs. Oats alone really are not complete nutrition for a bird. Their bodies are designed for a much more varied diet. Are you planning on free ranging? Feeding scraps? Whole oats have relatively high protein for a grain so it can make a good supplement but I would highly recommend feeding an actual chicken feed or mixing your own feed out of multiple grains that result in complete nutrition taking into account protein, calcium, amino acids, fiber, vitamins, etc. Without the proper amount of calcium your birds are likely to lay shell-less eggs which can cause health issues if it happens frequently.
Why are you not wanting to feed a complete nutrition feed? Cost? Concerns about quality? Allergies?
If the issue is concerns about quality/organic/GMOs/etc. then there really are a lot of different feed options on the market and there are some high quality organic feeds. I personally feed a soy-free, corn-free organic feed with a fish meal protein. Because these organic feeds tend to be on the expensive side I buy a higher protein feed and then supplement their diet with cheaper organic whole grains (wheat and barley) which I sprout as fodder. I choose the higher percentage protein so that I can add these lower protein sprouted grains without lowering the overall total protein intake too low. I feed a grower feed (as opposed to a layer that some people use. Layer has calcium mixed in for adult laying birds) so I give oyster shell separate from the feed as a calcium source.
If this issue is cost there are a wide variety of feed types with different protein bases and the generally the standard soy based feeds are much cheaper. You can even further lower costs by free ranging, supplementing in grains, growing fodder, fermenting feed, farming mealworms or other insects, etc.
You can make your own feed mix. There are a number of recipes on the internet that people have experimented with. I have not tried any so I can not recommend a particular one. I would do a bit of research to make sure you are get a good mix with the nutritional percentages you want in the end. The right balance of nutrition effects the health and egg production of the birds as well as the color and quality of the egg yokes.
If you do go with a feed make sure that you buy an age appropriate feed. There are lots of options that can work for young birds but laying feed is for laying birds only and should never be given to young birds.
