Kottaras, who was designated for assignment by the Brewers on Thursday, will report to Oakland on Monday, at which point a corresponding roster move will be made.
With catchers Kurt Suzuki and Derek Norris already on the roster, Kottaras' role with the A's is unclear.
"We'll get together and talk about that," A's manager Bob Melvin said. "I really don't know much about him. I think his on-base is around .400, so we'll probably delve into that a little bit more."
The 29-year-old left-handed hitter was batting .209 with a .409 on-base percentage, three home runs and 12 RBIs in 86 at-bats this year. In parts of five Major League seasons, Kottaras has hit .221 with a .326 on-base percentage, 18 home runs and 65 RBIs in 507 at-bats and 222 games.
The A's may opt to make another move in the near future to clear up the logjam at catcher. Suzuki has been the subject of multiple trade rumors, while Norris could be optioned to Triple-A Sacramento.
Norris, who started on Sunday, has a .195 average, three home runs and 11 RBIs. He has been a streaky player since his callup on June 21. After going 10-for-30 with a pair of homers in his first eight games, Norris endured an 0-for-30 slump, but had since logged five hits in his past 18 at-bats.
The A's had high hopes for De Los Santos, a 26-year-old reliever who was signed from the Dominican Republic in 2006. But after showing flashes in 2011, the right-hander struggled out of the gate in 2012, giving up seven hits and three walks in only three innings.
De Los Santos was optioned to Sacramento, where he posted a 7.25 ERA with 49 hits, 16 walks and 43 strikeouts 36 innings.
Greg Luca is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.