O Day? Ahh, you must go to UTS!
And technically UNSW O Week is only Tue-Fri, Monday is the Commerce and Economics Faulty Welcom Day. But since so many people turn up and some yellow shirts are there to give tours it gives off the impression that O Week starts on Monday.
Since there are welcomes for rural and international students into the previous week, the start of O Week is kinda of like the start of WWII, some say it is in Decemeber 1941 when USA comes in, others say when Japan attacks Manchuria in the early 1930's, or even that it began when WWI began. But most say 1939, just as most say O Week begins on the Tuesday.
Anyway, O Week isn't for 1st years, it's for the O Week volunteers. The 1st years are there purely for atmosphere, just like the fans at football games are just atmosphere for the corporate boxes!