The changeling, or Indian boy that Queen Titania stole from an Indian king and replaced with another child, was a particularly sweet and beautiful child, as we see from Puck's line, "She never had so sweet a changeling" (II.i.23). Since the boy is so beautiful, Oberon is actually angry with the queen for adopting him because Oberon is jealous. Oberon wanted the boy for his own care because he wanted to train him to be one of his knights, as Puck explains in his line "[a]nd jealous Oberon would have the child / knight of his train, to trace the forests wild" (24-25). Oberon has asked the queen for the boy, but she refuses, which has made Oberon extremely angry, as Puck explains in the lines,"But she perforce withholds the loved boy / Crowns him with flowers, and makes him all her joy" (26-27). Oberon and the queen keep having such violent fights about the issue that the elves hide in acorns out of fear, as we see in the line, "[A]ll their elves for fear / Creep into acorn cups and hide them there" (30-31).
Hence, we see that the reason Oberon is angry over the adoption is that he is jealous and wants the boy for himself.