The Bold needs to be AT&T branded, and it must currently be running an AT&T released version of the handheld code.
OTSAL has been available on the Bold for a while now. 4.6.0.167 was capable of it.
You do NOT install the OS on the BES.
The BES' role in this process is merely to request the wireless update on behalf of the user.
I would suggest that you run through the documentation for wireless updates and it should explain the entire process to you in more detail should you need it.
http://www.port3101.org/featured-bla...using-bes.html
(As a side note, there have been some problems with wireless update for the new AT&T OS on BES versions prior to 5.0. You might not be able to get this working yet.)
See here:
OTASL - Official BES Provisioned Versions - Port3101.org : Your BES Connection