Omarthis, Ghostfire Initiate

Omarthis, Ghostfire Initiate's second ability will trigger any time you put one or more counters on another colorless creature. This might be due to a spell or ability resolving or a colorless creature you control entering the battlefield with counters.

You must ensure that your face-down spells and permanents can easily be differentiated from each other. You're not allowed to mix up the cards that represent them on the battlefield to confuse other players. The order they entered the battlefield should remain clear. Common methods for indicating this include using markers or dice, or simply placing them in order on the battlefield. You must also track how each became face down (manifested, cast face down using the morph ability, and so on).

Because the permanent is on the battlefield both before and after it's turned face up, turning a permanent face up doesn't cause any enters-the-battlefield abilities to trigger.