Or else: kẻo

Kẻo means "or else": it follows a piece of advice and names the bad thing that will happen if you ignore it. Mặc áo khoác vô, kẻo lạnh. Put a jacket on, or you'll get cold (in Hà Nội, mặc áo khoác vào). Advice first, then kẻo and the result.

What follows kẻo is always something to avoid: being late, getting wet, the food going cold. For a good result, use để (so that) instead.

Kẻo is understood everywhere, but in everyday Saigon speech people more often say không thì or không thôi: Đi lẹ lên, không thôi trễ đó! Hurry up, or we'll be late! Không thì is the plainest of these, and you'll hear it in Hà Nội too.

English "or" can't be translated word for word here. Hoặc offers two choices; it doesn't warn.