Here is what UofT says regarding the deadline, it applies to Ontario law schools, so it should apply to your case.
If you’ve received an offer of admission from another ONTARIO law school before April 1, then you should have until April 1 to respond (accept or decline) to that offer online at OLSAS.
Please note that at OLSAS:
no response to that Ontario offer by that deadline keeps your UofT consideration open, and it has no effect on our decision
a provisional acceptance of that Ontario offer keeps your UofT consideration open, and it has no effect on our decision
a firm acceptance of that Ontario offer ends your UofT consideration immediately, we will close your file.
If you’ve received an offer of admission from any NON-ONTARIO law school at any time, then your response to that offer has no effect on your UofT admission consideration.
Osgoode's website states more of the same.
From what I understand, you can provisionally accept your offer at Queens without it affecting any of your other potential offers. I would assume you would have to pay their deposit.
Honestly though you should be getting into Osgoode soon anyways so I wouldn't worry too much.