Cream butter; gradually add sugar, beating well at medium speed of an electric mixer. Add the egg, beating well.
Combine the flour, soda, salt, and cocoa; stir well. Add the flour mixture to the creamed mixture alternately with the water, beginning and ending with the flour mixture. Mix well after each addition. Stir in vanilla. Add oats and chocolate morsels; stirring well to combine.
Drop dough by heaping teaspoonfuls onto ungreased cookie sheets. Bake at 350 degrees for 10 minutes. Let cool slightly on the cookie sheets, then transfer to wire racks and let cool completely.