John R. Neill provided thousands of illustrations for the Oz books, so it's not too surprising if some of his images can be traced to other inspirations. Two very good examples of this were shown in the Autumn 1981 Baum Bugle, and are pictured on the left.
These two drawings were reworked for use in 1918's The Tin Woodman of Oz. Both of the original illustrations were created for a magazine article on child suicide, but with a bit of work, Neill has changed them into Oz characters. The young girl has been transformed into Princess Ozma, and the schoolboy has become Woot the Wanderer. It clearly involved quite a bit more effort to pull off the Woot transformation!