"Leader board" vs. "leaderboard"

Solution 1:

If I look up leaderboard as one word in OED, it returns a reference to leader board as two words, which I suppose says something about which they think is correct!

Searching the corpora yields 13 instances without a space in the British National Corpus (BNC) against nine with a space; and in the Corpus of Contemporary American English (COCA) it's 82 without versus 90 with. [I think; I'm not all that familiar with those tools]

I would say those results are inconclusive.

Solution 2:

A board by any other name:

It's really no different than blackboard. Inevitably, "leaderboard" will become increasingly more formally accepted.

If in doubt:

If in doubt, I'd hyphenate the two words:

leader-board

...or...

Leader-Board

Purpose of one-word usage:

From the usage I've seen online, the advantage of the one-word variant is concision when used as a (or part of a) title.

E.g.

1. URL structures: www.test.com/leaderboard
2. Link text: My Leaderboard
3. Title: Masters Tournament Leaderboard