British Airways, American Airlines and Iberia had their request for antitrust immunity approved by the US Department of Transport last night.
The decision paves the way for the airlines to collude legally on transatlantic fares and services.
Last week the trio was also given the go-ahead by the European Commission.
The deal puts BA on a level playing field with European rival carriers Lufthansa and Air France who both form part of antitrust-based alliances.
BA's chief executive, Willie Walsh, said: "This final approval is fantastic news for all three airlines and the oneworld alliance.
"We've waited 14 years to bring the benefits of the transatlantic joint business to our customers and level the playing field with the other two global alliances.
"As we have argued all along, the EU-US market is highly competitive and Heathrow's liberalisation in 2008 opened it up even further. We are delighted that the US and EU authorities have recognised this."
In return for the right to work together on networks and fares, BA, AA and Iberia will have to give up some of their valuable slots at Heathrow.
However, Virgin Atlantic, which has long been a vocal opponent of the tie up, said the US DoT's decision to approve antitrust immunity was bad news for consumers.
The airline's owner, Sir Richard Branson, said: "It is clear that in taking this decision, the US DoT has decided to put the interests of BA and AA before those of the flying public.
"As a result, millions of passengers on both sides of the Atlantic will suffer the consequences of this monster monopoly. This is a bad day for consumers.
"The lesson that big corporations wishing to create anti-competitive monopolies can draw from this debacle is to keep applying year after year until they finally find regulators and politicians willing to wave the application through - no matter what the cost is to consumers."
On the contrary, American Airlines chief executive Gerard Arpey, said: "This is a great day for the customers, employees and shareholders of American Airlines and our joint business partners in the oneworld alliance.
"By working collaboratively, we will enhance our product offerings, strengthen our route networks and better position our airlines to compete in an ever-changing global aviation marketplace."
And not to be left out, Iberia's executive chairman, Antonio Vazquez, said: "A new kind of collaboration between our three airlines will lead to better service levels for our customers.
"This means that our customers will have more destinations to choose from around the world, better scheduled travel times, better connections and more competitive fares.
"I am convinced that consolidation is the best and only way to succeed in the airline industry, and the approval we have received today to create a joint business is a very important step towards this consolidation process."
A statement issued by BA late last night said the airlines planned to launch the transatlantic joint business this autumn and would co-operate commercially on flights between the EU, Switzerland and Norway and the US, Canada and Mexico.
Oneworld carriers Finnair and Royal Jordanian also received antitrust immunity from the US Department of Transportation.