Is Online Gambling Legal in Ontario?

Yes, if two things are true.

You are 19 or older and inside Ontario when you play. And the site is OLG.ca or one of the 84 sites on iGaming Ontario's official list.

Who Decides Which Sites Are Legal?

Canada's Criminal Code lets each province run gambling itself (section 207(1)(a)). Ontario does this through two government agencies. iGaming Ontario looks after the private gambling sites, and OLG (the Ontario Lottery and Gaming Corporation) runs its own site, OLG.ca.

A private company that wants to offer gambling in Ontario first needs approval from the AGCO (the Alcohol and Gaming Commission of Ontario, which writes the gambling rules). Next comes a contract with iGaming Ontario. After that, the site is added to the official list of legal sites, the iGaming Ontario directory.

How Do You Spot a Legal Site?

Its name appears in the iGaming Ontario directory (OLG.ca is the one exception).
It checks that you are 19 or older and that you are in Ontario before you can play.
You will not see it advertise sign-up deals, because Ontario's advertising rules forbid that.

What Happens If You Are Outside Ontario?

Your account still works for signing in. Betting has to wait until you return to Ontario. The iGaming Ontario player FAQ says so.