Danny O'Brien, squeezed out last week as vice president and general manager of the Texas Rangers, landed on his feet yesterday as president and chief executive officer of the Seattle Mariners. Yep, he gets to bump heads with the Rangers in the same American League West Division (albeit under the ridiculous 1979 AL schedule, we hear tell, teams will play clubs more games outside their division -- seven times 12 -- than inside -- six times 13). O'Brien's appointment leaves Lou Gorman with his title of Mariners' general manager but lower on the totem pole... Not only is Richard Hayes Holt, new general manager of the Mariners' Alexandria Class A farm club here, coming off similar duty for the Toronto Blue Jays' Utica, N.Y., club, but the erstwhile Dukes' new field skipper, Bobby Floyd, has a claim to fame: traded from the Orioles to Kansas City in 1970 for Moe Drabowsky.

Keith Showell of Camp Springs, JuCo All-America outfielder after his.424 hitting for Prince George's Community College last spring, then drafted in the second round last week by Texas, has signed with the Rangers; reports to Florida spring training, then Class A assignment.... Obviously, Showell was easier to please than the No. 1 pick in the whole January draft, New York high school southpaw Neal Heaton. He and his agent-attorney-brother originally asked the Mets for $138,000, haggled down to $50,000, but rejected what Met G.M. Joe McDonald says is the final offer: $35,000 salary, $7,500 incentive bonuses, $8,000 college scholarship. "Guess they want it all in cash," McDonald sighed, as Heaton said he would go either to Oklahoma State or U. of Miami and wait till next draft.