One of three donors accused of giving excessive campaign funds to Oklahoma County Commissioner Brent Rinehart pleaded guilty to a misdemeanor this week and has agreed to cooperate in the case against Rinehart and his former campaign manager, prosecutors said.

Engineer Bob Larkin agreed to a three-year deferred sentence and a $1,500 fine for a misdemeanor count of giving more than $5,000 to a political candidate, sai...