OB allowed users to send in emails and they would be posted, which is not a high bar (lower than, say, learning a Usenet reader) and a fair number of people contributed. It's just that LW made it much easier and unsurprisingly got way more contributions. This apparently came as a big surprise to Eliezer (but not me, because of my long experience with Wikipedia; it was a bit of a Nupedia vs Wikipedia scenario to my eyes).