Zar Ojanen, Scion of Efrava — ruling 1

Zar Ojanen, Scion of Efrava's ability is a triggered ability, not an activated ability. It doesn't allow you to tap it whenever you want; rather, you need some other way of tapping it, such as by attacking.

Zar Ojanen, Scion of Efrava — ruling 2

If Zar Ojanen becomes tapped while casting a spell or activating an ability, that spell or ability resolves after Zar Ojanen's triggered ability has resolved.

Zar Ojanen, Scion of Efrava — ruling 3

Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.

Zar Ojanen, Scion of Efrava — ruling 4

The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.

Zar Ojanen, Scion of Efrava : règles, fonctionnement et interactions (MTG)

Zar Ojanen, Scion of Efrava 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

Domain — Whenever Zar Ojanen becomes tapped, put a +1/+1 counter on each creature you control with toughness less than the number of basic land types among lands you control.

How it works : Zar Ojanen, Scion of Efrava

Original English text — Wizards of the Coast / Scryfall

#1September 9, 2022· Wizards of the Coast
Zar Ojanen, Scion of Efrava's ability is a triggered ability, not an activated ability. It doesn't allow you to tap it whenever you want; rather, you need some other way of tapping it, such as by attacking.
No other card is named in this ruling.
#2September 9, 2022· Wizards of the Coast
If Zar Ojanen becomes tapped while casting a spell or activating an ability, that spell or ability resolves after Zar Ojanen's triggered ability has resolved.
No other card is named in this ruling.
#3September 9, 2022· Wizards of the Coast
Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.
No other card is named in this ruling.
#4September 9, 2022· Wizards of the Coast
The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.
No other card is named in this ruling.