While Puck is in Owen's form, he is unable to use any magic (other than to change himself back to Puck), and is even vulnerable to magical assault (such as Demona's turning him to stone by night) - also, if Owen were to actually be killed, Puck would die with him. This limitation is partly due to the general limitations of a Child of Oberon in a human body, but, more significantly, also due to his deal with Xanatos. Since Xanatos chose Owen's mundane services rather than a wish from Puck, Owen has to adhere to this, and cannot use his magical abilities as Puck to assist Xanatos in any way.
Owen's most distinguishing feature (beyond his close resemblance of Preston Vogel, of course) is his stone hand, which became that way when he dipped it into the Cauldron of Life ("The Price"). Because of his aforementioned deal with Xanatos, he cannot (or will not) undo that condition, so his hand remains stone permanently.
