An Oakland A's announcer copped to a mistake he says he made during a pregame segment ahead of Friday's game against the Kansas City Royals -- in which he used the n-word.

Glen Kuiper, a veteran, was on camera before the match kicked off, and he was describing what he and the A's broadcasting team had gotten into earlier in the day ... including visiting the Negro Leagues Baseball Museum and hitting up a barbecue hot spot there in town.

The problem ... instead of saying "Negro" he said "n*****" -- which came across very clearly over the airwaves, without any bleeping or censoring whatsoever. It came and went, while Glen and his in-booth partner didn't seem to acknowledge it or even really notice it.

This wasn't addressed until much later on in the game, right around the 6th inning ... when Glen came back on the air and issued an apology as soon as they came back from a break. He said he said something earlier in the day that didn't come out the way he intended.

At first, nobody knew what exactly he was talking about ... 'cause he wasn't specific.

Eventually, though, somebody tracked down the video ... and now, the A's say they're looking into this and considering next steps. A rep for the team announced his use of the slur was "unacceptable," adding ... "The Oakland Athletics do not condone such language. We are working to address the situation." Kuiper's in his 20th season of calling A's games.