The ODD was filing was for fewer than 250k cases a year. The FDA ruled that there were more than 400k cases of OM.
The trial is for FDA review and approval which only applies to the US. However, currently the US is the standard for most world drug markets and, FDA approval often leads quickly to other approvals such as EU. So, US market initially what counts as far as the FDA is concerned. Whence, the 400k number.
As for placebos, the selection is random. If B-OM works, I would not want to be placed in the placebo group, ouch.