Riley was awarded $3.95 million rather than his $4.25 million request. Arbitrators Richard Bloch, Steven Wolf and John Woods heard the case virtually on Tuesday.
O'Neill, a two-time Gold Glove winner, was awarded $3.4 million instead of his $4.15 million request. Mark Burstein, Brian Keller and John Stout heard the case Friday.
Riley set career bests last season with a .303 average, 33 homers and 107 RBIs. He played in 160 games, tying Braves shortstop Dansby Swanson for the National League lead.
Riley had a $590,500 salary last year and was eligible for arbitration for the first time. He is hitting .243 with seven homers and 14 RBIs in 29 games this season.
No statistics or evidence from after March 1 are admissible other than contract and salary comparisons, timing set when Major League Baseball and the players' association agreed to the deal that ended the lockout.
O'Neill set career bests last year with a .286 average, 34 homers and 80 RBIs, finishing eighth in NL MVP voting. He is hitting .213 with two homers and 19 RBIs this season.
In his fifth season in the majors, O'Neill has a .256 average with 57 homers and 157 RBIs. He made $604,700 last year and also was eligible for arbitration for the first time.
Sixteen additional players are eligible for arbitration, with hearings scheduled through June 24. Players scheduled for hearings include Atlanta outfielder Adam Duvall, pitcher Max Fried and Swanson, along with New York Yankees outfielder Aaron Judge, New York Mets pitcher Chris Bassitt, Kansas City outfielder Andrew Benintendi, Minnesota catcher Gary Sánchez and Philadelphia pitcher Zach Eflin.
Arbitration hearings usually are held during the first three weeks of February but were delayed by the lockout.