You control at least one additional nontoken creature.
You have at least one creature card in your graveyard.
Whisper does not have summoning sickness.
Steps
Activate Whisper by tapping it, sacrificing Great Oak Guardian and any other creature you control, targeting any other creature card in your graveyard.
Ashnod the Uncaring triggers, copying Whisper's ability, targeting Great Oak Guardian.
Resolve the copy of Whisper's ability, returning Great Oak Guardian from your graveyard to the battlefield.
When Great Oak Guardian enters the battlefield, it triggers, untapping all creatures you control and giving them +2/+2 until end of turn.
Resolve the original Whisper ability, returning the targeted creature from your graveyard to the battlefield.
Repeat.
Results
Infinite death triggers.
Infinite ETB.
Infinite graveyard recursion.
Infinite LTB.
Infinite sacrifice triggers.
Infinitely large creatures you control until end of turn.
Infinite untap of creatures you control except Whisper.