The former chancellor also fired a warning shot to Theresa May by saying MPs must get to see and debate the plan for Brexit.
And he criticised the target she set as Home Secretary to get net migration down to the tens of thousands, which he said should not include students.
Mr Osborne warned that Britain could face turbulent economic times because of the Brexit vote.
He said: “I sweated blood to turn the British economy around and I’m incredibly proud we got all these people into work, we made Britain the fastest growing economy in the West. But let’s wait and see what happens.
“The pound fell sharply, we are not going to pretend any more that that doesn’t mean the pound in your pocket isn’t worth any less. Britain is poorer as a result.
“And all of the forecasts, including the government’s own independent forecast, show the economy is going to slow next year.”
He said it was in Britain’s “national interest” to have the “closest possible relations” with its key European neighbours, including the French and the Germans.
And the former chancellor urged the government not to discard existing free trade deals in Europe in the search for new ones elsewhere.
He said: “You can’t say we are a beacon of free trade in the world and the main thing you achieve is a huge act of protectionism, the biggest in British industry.”
Britain needs a “hard- headed assessment” about issues such as whether to leave the customs unions, he said.
And he urged the Prime Minister not to have “red lines” on issues like immigration.
He said: “I would say we are leaving the EU – that’s the only red line I would draw – let’s go in there and try and get the best deal for Britain.”