The classification and final analysis of the 39 events detected by Virgo and LIGO during the "O3a" data collection from April 1 to October 1, 2019, were made public on Thursday, October 29, 2020 on the arXiv preprints server. Twenty-six of these events were detected in real time and have been the subject of public alerts while the remaining thirteen are new. In addition to the new edition of the LIGO-Virgo event catalog, three articles were posted simultaneously: astrophysical properties of the catalog sources, general relativity tests and the search for coincidences between gravitational waves and gamma-ray bursts. Most of the O3a sources are black hole fusions; The study of their characteristics challenges certain theoretical models and opens up new perspectives. A probable merger of a binary system of neutron stars and two possible "hybrid" systems - consisting of a black hole and a neutron star - are also on O3a's hunting chart.