This tells you nothing about Richard's history, but here's one datapoint from another old-timer: I think I first encountered OB via Tyler Cowen's Marginal Revolution blog, and in particular this post from 2007-08-25. (I remember being struck by Tyler's suggestion that OB, despite its name, was actually exemplifying bias in a good way.) Weak evidence in favour of this being right is that my first comment seems to have been in 2007-09 and that this one in 2007-10 says I've been reading "for a month or thereabouts".

[EDITED to add: yes, different username.]