I'm going to answer even though I feel this might be found elsewhere and thus considered "basic", but if someone else can find such evidence on SCN, please let me know as I couldn't from a basic search.
The _OB module is the object buffer for the one order framework. You should not use that function module to get data. You should instead use the _OW module which is higher logical layer above that is also called by the the BOL and BAPI code.
I forgot the exact technical name for the _OW layer, but it's at the highest level of the one order business logic layer before you start crossing over to the UI layer of CRM. You should however not call any one order function module from the CRM Genil/BOL layer though.
Take care,
Stephen