@ViewScoped
public class UserForm extends BasePage implements Serializable {
    public void getOrgRoles(){
            log.debug("The DOM changed.");
    }
}

<div class="form-group">
          <h:outputLabel styleClass="control-label" for="organisation" value="#{text['organizationList.organization']}"/>
          <h:selectOneMenu value="#{userForm.user.organizationId}" id="organisation" styleClass="form-control" style="height: 38px">
          <f:ajax event="change" listener="#{userForm.orgRoles}" execute="@this"/>
          <f:selectItems value="#{userForm.orgs}"/>
          </h:selectOneMenu>
          <p:message for="organisation"/>
       </div>

<div class="form-group">
<h:outputLabel styleClass="control-label" for="organisation" value="#{text['organizationList.organization']}"/>
<p:selectOneMenu value="#{userForm.user.organizationId}" id="organisation" styleClass="form-control" style="height: 38px">
<f:selectItems value="#{userForm.orgs}"/>
<p:ajax event="change" listener="#{userForm.orgRoles}">
</p:selectOneMenu>
<p:message for="organisation"/>
 </div>