Giant Oyster — ruling 1

The activated ability can target only a tapped creature.

Giant Oyster — ruling 2

Activating the ability creates a continuous effect that works only as long as Giant Oyster remains tapped, a repeating delayed triggered ability that’s in effect only as long as Giant Oyster remains tapped, and a delayed triggered ability that triggers when Giant Oyster untaps or when it leaves the battlefield.

Giant Oyster — ruling 3

Giant Oyster’s ability can be activated during your upkeep, and the creature will get a -1/-1 counter during the draw step that follows it.

Giant Oyster — ruling 4

If the Oyster untaps during the draw step before its ability to add a -1/-1 counter resolves, the -1/-1 counter effect resolves and adds a -1/-1 counter after the -1/-1 counters are removed due to being untapped.

Giant Oyster : règles, fonctionnement et interactions (MTG)

Giant Oyster 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

You may choose not to untap this creature during your untap step. : For as long as this creature remains tapped, target tapped creature doesn't untap during its controller's untap step, and at the beginning of each of your draw steps, put a -1/-1 counter on that creature. When this creature leaves the battlefield or becomes untapped, remove all -1/-1 counters from the creature.

How it works : Giant Oyster

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2006· Wizards of the Coast
The activated ability can target only a tapped creature.
No other card is named in this ruling.
#2September 25, 2006· Wizards of the Coast
Activating the ability creates a continuous effect that works only as long as Giant Oyster remains tapped, a repeating delayed triggered ability that’s in effect only as long as Giant Oyster remains tapped, and a delayed triggered ability that triggers when Giant Oyster untaps or when it leaves the battlefield.
No other card is named in this ruling.
#3September 25, 2006· Wizards of the Coast
Giant Oyster’s ability can be activated during your upkeep, and the creature will get a -1/-1 counter during the draw step that follows it.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
If the Oyster untaps during the draw step before its ability to add a -1/-1 counter resolves, the -1/-1 counter effect resolves and adds a -1/-1 counter after the -1/-1 counters are removed due to being untapped.
No other card is named in this ruling.