At once and On-the-spot

At once adverb - Without delay or hesitation; with no time intervening.

On-the-spot and at once are semantically related. In some cases you can replace phrase "On-the-spot" with "At once", this phrases are similar.

On-the-spot adjective - At the scene of action.

At once and on-the-spot are semantically related. Sometimes you can use "At once" instead an adverb phrase "On-the-spot".