So why are all the O2 venues called O (letter) 2 (number)?

I thought it should be 02 (both numbers) for year 2002, and O2 came to be the accepted way here, but that does not follow exactly as first show dates under these new venue names only start after 2006 and onwards.

Could it O!!, and then second name for the venue; ie Millenium Dome = O2 London??