As the first available prebiotics for neonates, milk oligosaccharides regulate gut microbial composition and modulate host immune response, playing a crucial role in the holobiont assembly. By using two livestock models (pigs and rabbits) with different maturity levels at birth, HoloOLIGO aims to decipher causal links between milk oligosaccharidesstructures, the offspring microbiota and immune system. We will create a database using data mining of the literature to find, visualise and analyse milk oligosaccharidesstructure diversity patterns within and between mammalian species. We will produce the first MO data in rabbits and expand them in pigs. To understand structure importance of milk oligosaccharides, we will undertake in vitro functional analyses in both species on commensal bacterial strains and intestinal immune cells and further validate results in vivo. Finally, we will evaluate, via in silico analyses (pig) and in vivo (rabbit), the existing genetic variability and assess the genetic determinism of milk oligosaccharidescomposition.