JUNO is a 20 kt liquid scintillator detector under construction in Jiangmen, China, whose goal is to determine the neutrino mass hierarchy. Its data taking is expected to start in 2022. In order to meet the stringent requirements on the radiopurity of the liquid scintillator, the OSIRIS pre-detector is being designed to monitor the liquid scintillator during the several months of filling the large volume of JUNO. OSIRIS will contain 20 ton of scintillator and will be equipped with 76 20-inch PMTs. The data acquisition system will have no global hardware trigger: instead, each PMT will provide a data-stream composed of the digitized PMT pulses, each containing a time stamp. Based on the latter, dedicated software will organize these data streams into events. This talk will discuss the optimization of the event trigger conditions, for the inner liquid scintillator detector as well as outer water Cherenkov detector, considering the expected rates of different radio-active contaminations, cosmogenic muons, and the PMT dark rates
