I was surprised that the 2006 version of this page uses usama and not osama:
http://classic-web.archive.org/web/20060310035124/www.fbi.gov/wanted/terrorists/fugitives.htm
IIRC back when this started, there was inconsistency with Bin Laden’s name.
Ben Ladin, Bin Laden, Bin Ladin, Osama, Usama.
Dick Cheney tried to be consistent, but it seems that the MSM focused on the name Osama, so it stuck.
Look at what happened with Ghaddafi. (spelling apparently optional)
Seems to me the WH is NOW using Usama because Osama is too close to Obama.