As soon as and At once

As soon as conjunction - Just at the moment that.

At once and as soon as are semantically related in once topic. In some cases you can use "At once" instead an adverb phrase "As soon as".

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

As soon as and at once are semantically related in once topic. You can use "As soon as" instead an adverb phrase "At once".