Please, check this url: https://salesforce.stackexchange.com/questions/45944/retriving-openevent-details-using-subscriberkey
with this example:

    RetrieveRequest retrieveRequest = RetrieveRequest.Factory.newInstance();
    retrieveRequest.setObjectType("OpenEvent");
    retrieveRequest.setPropertiesArray(new String[] { "ObjectID", "SendID", "SubscriberKey", "BatchID" });
    
    SimpleFilterPart filter = SimpleFilterPart.Factory.newInstance(); 
    filter.setProperty("SubscriberKey"); 
    filter.setSimpleOperator(SimpleOperators.EQUALS); 
    filter.setValueArray(new String[] { "[email protected]" }); 
    
    retrieveRequest.setFilter(filter);