An ot is a sign, and the Exdous story abounds in them. From Moses's initiation at the burning bush to the splitting of the Red Sea, God took the Israelites out of Egypt with otot and moftim (signs and portents).
To assuage Moses's initial fears, God tells him: "I will be with you, that shall be your ot that it was I who sent you"(Exodus 3:12). According to Exodus Rabbah (3:4), the ot was God's commitment to support Moses. Others point to the burning bush as the ot - his future trials will not consume Moses and God will be in the flames with him.
It is only after his further protestations that God gives Moses the supernatural otot of the staff transforming into a snake and the leprous hand. Perhaps this is a hint at their being a concession to Moses's fear of failure.
The most enduring ot that God gave the Jewish people is the Shabbat. "Nevertheless you must keep my Shabbatot, for this is an ot between Me and you" (Exodus 31:13). Our commitment to turn away from the prosaic and dedicate a day a week to spirituality is a constant reminder that God is always with us.