Miles, an Antioch High product who had just been acquired from the Cubs on Dec. 3, was shipped to Cincinnati in exchange for infielder Adam Rosales and outfielder Willy Taveras.
The A's also claimed minor league infielder Steve Tolleson off waivers from the Minnesota Twins.
It's obvious Rosales, 26, was the key player for the A's in the Reds trade, as Taveras was immediately designated for assignment -- along with left-hander Dana Eveland -- in order to clear space on the 40-man roster.
The A's later designated infielder Gregorio Petit for assignment to clear a spot on the 40-man roster for Tolleson.
Oakland had been searching for a backup infielder even after acquiring Miles, an indication the seven-year veteran might not be with his hometown team long.
Rosales played all four infield positions while batting .213 with four home runs and 19 RBI with Cincinnati last season, and the A's were targeting a player who could play shortstop behind Cliff Pennington. Miles, 33, has played 115 major league games at short but is primarily a second baseman.
"We had the ability to get a player who's younger and will be here a while, which is something we were interested in," A's GM Billy Beane said.
The A's aren't likely to keep Tavares considering their outfield depth. If they aren't able to trade him and he goes unclaimed on waivers, Oakland is likely to release him and eat his $4 million salary for 2010.
Miles will make $2.7 million this season, of which the A's were on the hook for $1.7 million. So in essence, the A's have paid $2.3 million for Rosales' rights.
Although the A's were linked to Johnny Damon in speculation, they signed another free agent outfielder in Gross, who got a one-year, $750,000 deal. Gross, 30, hit .227 with six homers and 36 RBI in 282 at-bats last season for Tampa Bay.
He can play all three outfield spots and figures to provide a left-handed hitting option off the bench. Beane pointed to the fact that the A's traded outfielders Scott Hairston and Aaron Cunningham for third baseman Kevin Kouzmanoff, then saw promising prospect Grant Desme retire as reasons he looked for outfield depth.
"Gross is a versatile guy we've had interest in in the past."
Beane said Gross' addition doesn't lessen the chances of top outfield prospect Michael Taylor playing his way onto the big league roster. But he likes the fact that the A's have added depth this winter that allows them not to rush their best prospects.
"When you get to this level, you go with the best you have," Beane said. "I think we've balanced our plate pretty good. Taylor, (Chris) Carter, (Adrian) Cardenas, (Jemile) Weeks - it still probably wouldn't hurt them to get more time in Triple-A if it looks like it's needed."