SIN you can get right away with no problems, just go to nearest government office after landing.
If you have not established residency in Ontario, you may be ineligible to even register for an OHIP card.
When applying for OHIP card you must prove that you have Ontario residency. They will only accept certain specific documents for you to prove this. Take a look at "List 2" from here:
http://www.forms.ssb.gov.on.ca/mbs/ssb/forms/ssbforms.nsf/GetFileAttach/014-9998E-82~1/$File/9998-82E.pdf
If you don't have 1 of those specific documents that shows your Ontario address, you will not be able to register.
Regardless, if you go back to US for a year after landing, you won't be eligible for OHIP coverage until 3 months after you permanently move back to Canada. So there is no rush, just wait until you actually move back here to apply for the card.