Why did slaves learn to read if it was outlawed by slave codes

The slaves did it Illegally, and eventually learned to read and write. Some free men taught the slaves and then the slaves taught each other. Whether the man that taught them got caught? Who knows. Also note, Christian owners wanted their slaves to know how to read the Bible. They were people too, and they had their own individual right to learn to read and write on their own will. It was against the law at the time.

Ending prohibition in oklahoma schools