Onslaught — ruling 1

A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.

Onslaught — ruling 2

Can target an already tapped creature.

Onslaught — ruling 3

The ability is not optional. If you have the only targetable creatures, you have to tap one of your own.

Onslaught : règles, fonctionnement et interactions (MTG)

Onslaught is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you cast a creature spell, tap target creature.

How it works : Onslaught

Original English text — Wizards of the Coast / Scryfall

#1April 1, 2008· Wizards of the Coast
A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Can target an already tapped creature.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
The ability is not optional. If you have the only targetable creatures, you have to tap one of your own.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Onslaught.

Monstrous Onslaught

Monstrous Onslaught deals X damage divided as you choose among any number of target creatures, where X is the greatest power among creatures you control as you cast this spell.