Oneofthesub-systemsoftheTrigger/DAQsystemofthefutureATLASexperimentistheOnlineSoftwaresystem.Itencompassesthefunctionalityneededtoconfigure,controlandmonitortheDAQ.ItsarchitectureisbasedonacomponentstructuredescribedintheATLASTrigger/DAQtechnicalproposal.ResularintegrationtestsensureitssmoothoperationintestbeamsetupsduringitsevolutionarydevelopmenttowardsthefinalATLASonlinesystem.Feedbackisreceivedandreturnedintothedevelopmentprocess.Studiesofthesystem.behaviorhavebeenperformedonasetofupto111PCsonaconfigurationwhichisgettingclosertothefinalsize,Largescaleandperformancetestsoftheintegratedsystemwereperformedonthissetupwithemphasisoninvestigatingtheaspectsoftheinter-dependenceofthecomponentsandtheperformanceofthecommunicationsoftware.Ofparticularinterestweretheruncontrolstatetransitionsinvariousconfigurationsoftheruncontrolhierarchy.Forthepurposeofthetests,thesoftwarefromotherTrigger/DAQsub-systemshasbeenemulated.Thispaperpresentsabriefoverviewoftheonlinesystemstructure,itscomponentsandthelargescaleintegrationtestsandtheirresults.
