Assembly factor for cytochrome c oxidase (respiratory chain complex IV) (PubMed:21457908, PubMed:35750769). Stabilizes an early formation of cytochrome c oxidase assembly factors, until it is displaced by the metallochaperone copper-delivery protein COX17 (PubMed:35750769). {ECO:0000269|PubMed:21457908, ECO:0000269|PubMed:35750769}. This is the function of COA5 (cytochrome c oxidase assembly factor 5, Ensembl gene identifier ENSG00000183513).