Seems to me that the servers running the latest beta patch are more stable. Might not be the case, but either way you guys should install it, as it fixes known ArmA2 bugs. Plus you'll have more servers you can join. Servers running on those patches are usually not full, so that's good.
Anyway, people have so different A2+OA setups that they run into problems when trying to get DayZ working with the patch. I think I've got a universal fix for most problems.
1) Download latest beta patch - http://www.arma2.com/beta-patch.php (zip file in the top green box)
2) Install the patch, should be easy enough.
3) Download the tiny zip file from the attachments of this post. Extract it and put the .cmd file inside to your Operation Arrowhead folder.
4) Launch your game from that file. (run DayZ+beta.cmd)
If everything goes well, you will see it saying "ArmA2 beta expansion @dayz" on the right side of the screen when in the main menu.
There's a similar file in your OA folder already, all I basically did was add a switch to launch DayZ with beta, instead of regular Combined Operations. You can edit those files with any text editor and add your own additional switches and whatnot if you want.