Without the pretense of objectivity, the fundamental journalist's obligation of factual accuracy would remain. Opinion journalism brings new ethical obligations as well. These can be summarized in two words: intellectual honesty. Are you writing or saying what you really think? Have you tested it against the available counterarguments? Will you stand by an expressed principle in different situations, when it leads to an unpleasing conclusion? Are you open to new evidence or an argument that might change your mind? Do you retain at least a tiny, healthy sliver of a doubt about the argument you choose to make?
Much of today's opinion journalism, especially on TV, is not a great advertisement for the notion that American journalism could be improved by more opinion and less effort at objectivity. But that's because the conditions under which much opinion journalism is practiced today make honesty harder, and doubt practically impossible.