have you rebooted since installing OB? a long shot, but this is the first main problem people have
do you have OB set to run trayserver? you can turn this off in the settings. if this is turned off, then the system tray wont get any items
if you have got past those two problems, then do you run DesktopX ? i have occasionally heard of conflicts between OB and DX over the system tray, but they should have been long since fixed.
i do know that 1.5 works on win2k professional, i ran it myself with great success.
do you actually have a system tray item in your theme? and is it visible on the screen?
if so, check the settings for auto hiding icons. you can ask OB to hide system tray icons that haven't been updated / used for 5 minutes.
if it is none of these, then if the theme is small (56k connection) and you can wait a few days for me to get around to it, you can email me the theme and i will have a look for you.
no promises, but i have played with OB for a while, so i may be able to help