302.3. No player receives priority during the untap step, so no spells or abilities can be played or resolved. Any ability that triggers during this step will be held until the next time a player would receive priority, which is usually during the upkeep step.
My friend tells me mesmeric Orb triggers in the untap phase, and you can't play anything during that phase. But I say you can because why else would they give it a triggered ability if you can't respond to it?