Mwonvuli Ooze — ruling 1

If you choose not to pay Mwonvuli Ooze’s cumulative upkeep, it will still get an age counter before being sacrificed. Its power and toughness will momentarily increase before being put into the graveyard, which might matter to certain other cards (such as Proper Burial).

Mwonvuli Ooze — ruling 2

Its power/toughness changes when the cumulative upkeep resolves, not when it triggers.

Mwonvuli Ooze : règles, fonctionnement et interactions (MTG)

Mwonvuli Ooze 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

Cumulative upkeep (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay for each age counter on it.) Mwonvuli Ooze's power and toughness are each equal to 1 plus twice the number of age counters on it.

How it works : Mwonvuli Ooze

Original English text — Wizards of the Coast / Scryfall

#1April 1, 2008· Wizards of the Coast
If you choose not to pay Mwonvuli Ooze’s cumulative upkeep, it will still get an age counter before being sacrificed. Its power and toughness will momentarily increase before being put into the graveyard, which might matter to certain other cards (such as Proper Burial).
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Its power/toughness changes when the cumulative upkeep resolves, not when it triggers.
No other card is named in this ruling.