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. Decay of radioactive isotopes in the liquid scintillator can mimic neutrino signal events. In order to meet the stringent requirements on the radiopurity of the liquid scintillator, the OSIRIS (Online Scintillator Internal Radioactivity Investigation System) pre-detector is being designed to monitor the liquid scintillator during the several months of filling the large volume of JUNO. OSIRIS will contain 18 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 by sorting the time stamps and applying trigger logics. To optimize the trigger conditions, physics events are generated from the Geant4-based OSIRIS simulation software. Then the output of photon hits on all PMTs are transferred into our DAQ simulation software where dark counts are also simulated. Afterwards, different trigger conditions are applied in the event builder software. This poster will show 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.