No information for adults; reported stomach contents (n = ?) are insect parts and grit (Schulenberg et al. 1984), but Olive Finch probably also consumes some seeds. For food items provisioned to nestlings, see Reproduction.
Voelker, K. N. and D. L. Slager (2015). Olive Finch (Arremon castaneiceps), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.olifin1.01