The OAG is definitely your best bet. One year, 12 issues, $339. Subscribe every two or three years and you keep up with the changes, especially if all you want to know is what airlines serve an airport. It has every scheduled flight in the country. And after you've used it for a while, you'll convert to a 24-issue subscription every year and love to be able to save bundles of time by calling your travel agent (or getting on line) and telling him/her/it where you want to go, when, and on which flight.
BTW: Back when I was trying to achieve status on DL, I used the OAG to construct my own connections. I'd go DCA-CVG-ATL round trip and get four segments instead of two - and a few extra miles, as well. No travel agent or conputer res system would ever suggest that.

[This message has been edited by philforest (edited 05-11-99).]