I very much disagree with the comment presented here. His argument has some merit: you've cast Oath of Nissa, so clearly you need a land in play. Then he disregards the contents of your hand as irrelevant. Well, let's back up a minute.
You've kept the hand. We're already dealing with a conditional probability statement here.
Given that you've kept the hand and
given that you're not terrible at Magic, we may assume that you kept a hand with at least one mana source. Whether that mana source is green is a complicated question and depends on the deck in question, but the fact remains that having at least one land in play doesn't really tell us anything about our opener. We could have kept a 1-lander or a 6-lander, the only possibility that is eliminated is that the player kept a no-lander. Given that the mulligan rule exists, we can rule out this possibility. (In either case, we've assumed
From this perspective we haven't gained any information regarding the contents of our hand. However, the observation that we will never keep a no-lander may have some minor impact on our distribution of hands, but given that the quantity in question is Lands + Creatures + PW, there's not a clear way to use this information, and certainly, not in a way that is easily communicated to a general audience.
I appreciate any and all comments I get on my articles, but this type of comment puts me in a rough spot. Either I "pull rank" with my own credentials and post a long thing refuting it, or I leave it as some sort of tacit admission that my computations are incorrect.