Omnath

[Kaworu, the Fifth Child]

Does Omnath keep mana in the pool forever, or just until end of turn?

[Nymuera]

Does {Omnath, Locus of Mana} keep mana in the pool forever, or just until end of turn?

Indefinitely, it stays there until it is spent or the creature leaves the battlefield. Also it can be spent at any time, even several turns after it was put there.

[ibtrickey]

What constitutes having it in your mana pool? Tapping it and not using it?

[Splicer]

That would work, also things like {Burning Tree Emissiary} would do the trick

[FustyDavorite]

What constitutes having it in your mana pool? Tapping it and not using it?

pretty much. Tap a forest and if you don't use the mana there's {G} sitting in your mana pool.

[particle]

What constitutes having it in your mana pool? Tapping it and not using it?

actually just tapping mana adds it to your mana pool. then you use it, so whether or not it gets used it always starts in your pool. for things like {doubling cube} and {glissa sunseeker}.

[ibtrickey]

Okay=]
This helps me out a lot because I have the card and was not sure exactly