OAA is a cumulative stat, not a rate stat.
For 2025 Larnach is -4 OAA in 441 innings (-2 Total Zone, -5 DRS), Wallner is -4 OAA in 654 innings (-6 Total Zone, -6 DRS). On a rate basis, Wallner is slightly better. Larnach is also getting compared to LF, who are generally worse fielders than RF. That is a point for Wallner.
Neither one is good, but Wallner actually rates marginally better by the defensive stats.