Hi Mery,
You have a couple of options. I mention the "proof of funds requirements for each, since as a graduate student, it may be an issue - most don't have a lot of money

1) FSW3: If you have accumulated 1950 hours as a TA (equivalent of 1 year of full-time employment), you can apply under the federal skilled workers program, Category 3. (It's the 2nd bullet point here for some reason, but it's FSW3).
http://www.cic.gc.ca/english/immigrate/skilled/apply-who.aspYou need to show "Proof of settlement funds" (I know, you're already settled, don't get me started..), basically, savings of around 11K if you're single, no kids.
2) CEC: Because you got your MA in Canada, your work since then is considered post-graduation experience under the Canadian Experience Class (CEC). If you accumulate 1950 hours as a TA/RA in your PhD program within 2 years, you can apply under the CEC program. (Alternatively, 3900 hours in 3 years as a Skilled worker, not student). No proof of funds requirement. But getting the hours may be difficult...
http://www.cic.gc.ca/english/immigrate/cec/index.asp3) PNP: Good thing you mentioned Ontario. If you earn a PhD in Ontario, you automatically qualify under the Provincial Nominee Program (PNP), Opportunities Ontario.
You can apply as soon as you've completed all program requirements. The downside - it costs 2000 extra for the Ontario nominee application (on top of the already expensive Skilled Workers application), and you guessed it, you still need to show proof of funds

It's the most expensive option, you have to first apply to Ontario and then to CIC, but it's there. At the moment.
http://www.cic.gc.ca/english/immigrate/provincial/apply-who.aspKeep in mind that rules and eligibility criteria frequently change...