From the times I’ve seen it I’m guessing it comes last in a list, but I’m still curious as to:
[ul]
[li]When it started[/li][li]How it started[/li][li]Maybe even Why it started[/li][li]Hi Opal! (If I used “Hi Opal!” incorrectly, I apologize)[/li][/ul]
Opal is a longtime poster here. (She has her own forums which are well worth visiting.) Back in the mists of antiquity, she jokingly protested lists which only included two items (ie, if you only have two items in a list, just put them in a sentence instead of listing them bullet-point style).
In response, many Dopers started teasingly posting two-point lists with a third point consisting of “Hi Opal!”
This eventually degenerated into a custom wherein a list of any length would have as its third point “Hi Opal!”
Many oldtimers here (and this is an old board) consider the joke to be well past retirement and consequently it is not often seen anymore and is probably best left to rest in peace.
Sorry if I sounded stuffy there, Achren.
I should add:
Welcome to the SDMB!
I’m embarrassed. Just today I discovered not only Opal’s board, but also where the answer to my question is. (Here, for the curious.)
Apologies to all!
Hoops thanks for my first official welcome. 
Until today, I had no idea how truly old this board was. I always thought it started mid 1998/1999. Never heard of the AOL days until today.
(ignorance fought and conquered)
For the answer to all (or at least many) things historical about the SDMB, visit this thread.
Be warned - don’t go in there unless you have a lot of free time. There’s links to many fascinating threads in there, you can easily spend several hours going through all the links.
As an added FYI, I once tricked a number of newbies into believing that I started the “Hi Opal” phenomenon. In reality, it started before I joined this board.