Your academic orientation on the 23rd is the one you book. This is pretty much your first proper welcome and introduction to the universities services and facilities, you'll meet other newbies, get a guided tour of the uni, get some free food and other cool stuff.
O-week is just the first week back at uni. Since the realisation that our massive three month holiday is over is so damn depressing, we pretty much have a big week long party to make ourselves feel better. There's music and club/society marquees and drinking etc. So yeah, you will end up going to both.