Although they have not thrown an MLB pitch in 2023, Riley O'Brien threw 728 pitches that were tracked by the PITCHf/x system between 2021 and 2022, all of them occuring in the MLB Regular Season. In 2022, they relied primarily on their Slider (81mph), Fourseam Fastball (94mph) and Sinker (95mph). He also rarely threw a Change (87mph).
In 2022, compared to other RHP:
His slider is a real worm killer that generates an extreme number of groundballs compared to other pitchers' sliders, sweeps across the zone, has exceptional depth and has below average velo. His fourseam fastball is a real worm killer that generates an extreme number of groundballs compared to other pitchers' fourseamers, has essentially average velo and has slightly less natural movement than typical. His sinker generates an extremely high number of swings & misses compared to other pitchers' sinkers, has well above average velo, results in somewhat more groundballs compared to other pitchers' sinkers and has some natural sinking action. His change (take this with a grain of salt because he's only thrown 2 of them in 2022) is basically never swung at and missed compared to other pitchers' changeups, has an obvious armside fade, results in more flyballs compared to other pitchers' changeups and is slightly firmer than usual.