When adding events to events.xml you'll need to browse the existing events to make sure you aren't adding a duplicate. Here are a couple of special searches you can use to look through the events file a bit more rapidly:

1) If you want to find all events that take place in, say, Crete, you can run an XPath search (there's a search field at the top-left of oXygen) to do it. That search looks like this:

//event[.//placeName[@corresp='places.xml#crete']]

Paste the above line in its entirety in to the XPath search field and hit 'Enter'.
To search for another place, change crete to the xml:id of the place you want to find - leave the quotes alone!

2) Something similar can be done for characters:

//event[.//persName[@corresp='characters.xml#eurystheus']]

Again, paste the above line in to the XPath search field and hit 'Enter'

3) You can combine this functionality, too. To find all events which include Eurystheus and Mycenae, try this:

//event[(.//persName[@corresp='characters.xml#eurystheus']) and (.//placeName[@corresp='places.xml#mycenae'])]

That's all one line, and it goes in that XPath field.