Excellent question about why BA didn't do this in the first place and avoid the start-up costs and BALPA disputes associated with OS....
I don't know the answer to this question, but looking forward I have significant doubts that the OS project will be hugely successful...Somehow it just doesn't seem to hang together correctly, particularly the Paris services....
Brussels might succeed as there is no ' home ' market competition to the USA, but service between Paris / Amsterdam / Frankfurt / etc and New York or other East Coast cities without a network for onward connections ( not even through or with OneWorld partners, of which I believe OS is not a member ) seems to restrict the services to a point-to-point market on routes that are already highly competitive with national carriers for local passengers and alliance hubs for onward connections....
As someone mentioned earlier with the question re Air Lib, has the current BA Board forgotten the last fiasco for BA in France - this country is a graveyard for most foreign investors due to the high degree of protectionism that the French Government indirectly provides to major French companies and what are considered as strategic industries....
And even with this purchase, BA is still not quite the nuisance to AF/KL that the combined DL/NW/AF/KL presence at Heathrow now represents to BA....It won't be long before the SkyTeam group turn Heathrow into a reasonable sized hub with huge numbers of codeshares for the North Atlantic market....Does OS have this capability ? I think not - no onward networks, no alliance for high yield FT's who are ALL mile collectors, Orly is not a major hub and never can be, and, most of all, France is France and all things French !
So curiouser and curiouser....