After less than a year of national competition, "Donahue" has been been dwarfed by "The Oprah Winfrey Show." According to the most recent A.C. Nielsen Co. ratings available for syndicated shows, Winfrey had an average 10.7 rating for February compared with Donahue's 7.9. Winfrey's victory came even though her show was seen in more than half the markets in the morning when audience levels are lower. By this fall, the show will air almost exclusively in the afternoon, so the ratings are expected to be even higher. Michael King, president of King World, the syndicator of Winfrey's show as well as the top-rated "Wheel of Fortune" and "Jeopardy," says advertisers buying time on "Oprah" in markets where it will be on in the afternoon this fall have been guaranteed a record 14.0 rating. The Chicago-based "Oprah" is seen in about 145 markets. "Donahue," which moved from Chicago to New York in 1985, is seen in more than 200.