Does flash allow you to cast a creature spell during the combat phase on an opponent's turn?

Flash (CR 702.8a) grants a creature spell the timing permission of an instant. Normally, creature spells can only be cast during your own main phase when the stack is empty and you have priority (CR 307.1). Flash overrides that restriction entirely.

This means a creature with Flash can be cast at any point you receive priority — including during your opponent's combat phase, in response to a trigger, or even after attackers are declared (CR 702.8a, 116.1).

One of the most powerful uses is casting a Flash creature during the declare blockers step. For example, your opponent attacks with a 3/3. You cast Briarbridge Tracker (a 3/3 with Flash) after attackers are declared, then declare it as a blocker, killing the attacker and leaving your creature alive.

You can also cast Flash creatures during the opponent's end step, which is a common timing trick to get a creature onto the battlefield as late as possible before your own turn — avoiding sorcery-speed removal windows your opponent might have on their main phase.

Keep in mind you still need priority to cast any spell (CR 116.1b). If your opponent has no spells or abilities on the stack and simply moves to combat, you will receive priority at each step and can use it to cast your Flash creature at the appropriate moment.