What program are you in? First years register for courses pretty soon I think. I could be wrong but I think Eng I/Nursing I are the 29th of this month? Not sure.
As for OSAP, do it as soon as possible (before the Queen Elizabeth II deadline, which should be on the site -- check your account). If you can get the tax form information from your parents tonight, do it tonight so you don't forget. OSAP is an easy thing to procrastinate.
No orientation is mandatory, and they're really not necessary since welcome week will teach you everything you need to know about where buildings are.
EDIT: Get on SOLAR to register for your courses by the date shown on here:
http://www.ece.mcmaster.ca/docs/SOLA...%20Final. pdf
And you can find the classes you need to register in by looking for your program in here:
http://registrar.mcmaster.ca /CALEN...11-12/pg3.html
You don't actually have to make your own timetable since SOLAR will make you one that doesn't conflict... but making a timetable is good practice to get an idea what your workload will be like. You can get class times from here:
https://adweb.cis.mcmaster.ca /mtt/