"/." is valid url, but this is not working in Opera. This is bug which should be fixed soon. But still you will have to wait until user will put space and the start typing text to omnibox. Event onInputStarted is the earliest event you can catch. I tried
chrome.omnibox.onInputStarted.addListener(function () {
chrome.tabs.create({url:"http://slashdot.org"});
})
and browser create tab after user type first letter.