How to load the list of items from p: selectOneMenu when clicking to open the dropdown?

You can change the get of the list in the bean:

public List<Bla> getListaBla() {
    updateBlaList();
    return this.blaList;
}

You can fire an ajax request that updates the list.

<p:selectOneMenu value="#{bean.object.property}" id="myId" required="true">
    <f:selectItems value="#{bean.list}" />
    <f:ajax event="click" listener="#{bean.updateList}" />
</p:selectOneMenu>

Not the ideal way to do it .. the best way would be to reload the page already bring all the items.

What if you include a poll? From time to time you update your select ...

<p:poll interval="3" listener="#{counterView.increment}" update="txt_count" />