The default Apex Change event is what you're looking for. On your Foex Combo item add a new dynamic action, it should default to the Change event, if you do not have any conditions for when the change event will fire just click next to move into the action choices.
When the user selects a value from the list this event will set the Combo page item to the value of the selection. You could then use this value in other parts of your page to do what you need. (e.g. Lets call the Combo item :P100_COMBO, you select 'test' from the combo(page item :P100_COMBO now holds the value 'test'), which then fires the Change event and proceeds to your specified action, one of your actions is to set another page item to this new value 'test' using a PL/SQL block; :P100_NEW_ITEM := :P100_COMBO, :P100_NEW_ITEM will now hold 'test'). Be sure to submit the Combo page item to the session and refresh any region/page items on the page that will be changed through your action.
I hope this helps.