The Observed Antibody Space database, or OAS, is a project to collect and annotate immune repertoires for use in large-scale analysis. It currently contains over one billion sequences, from over 75 different studies. These repertoires cover diverse immune states, organisms (primarily human and mouse), and individuals. The data has been sorted, cleaned, annotated, translated, and numbered, and we have made all of it available for download on this website. You can either download the entire dataset, or you can download a subset of sequences by using our search forms, which allow you to filter the sequences based on attributes such as chain type, species, and disease state. OAS now contains both unpaired and paired antibody sequences; click the links above to access the search forms for each. We aim to regularly update OAS with newly released BCR datasets, and hope this resource will facilitate data mining of immune repertoires for improved understanding of the immune system and development of better biotherapeutics. Please note that the data contained within OAS is available under a CC-BY 4.0 license.