Can a Mareva injunction be granted without notice to the other side in Ontario?

Yes, and in practice this is the norm rather than the exception for Mareva injunctions - the whole point of an asset freeze is often defeated if the defendant gets advance warning and has time to move or hide assets before the order takes effect. Ontario courts will grant a Mareva injunction on an ex parte (without notice) basis where the plaintiff can show genuine urgency and a real risk that notifying the defendant in advance would let them dissipate the very assets the order is meant to protect.

Because granting a significant order without hearing from the other side is a serious departure from the normal adversarial process, courts expect an especially rigorous application in return - strong evidence on the merits, clear evidence of dissipation risk, and complete, scrupulously honest disclosure of everything relevant, including facts that might hurt the plaintiff's own case. An ex parte Mareva injunction is also typically granted only for a short period, with a further hearing scheduled promptly where the defendant can appear and argue the order should be varied or set aside, so it's understood from the outset as an interim measure rather than a final, unchallengeable order.