Turkey is hosting the G20 finance ministers and central bank chiefs for the first time as it holds the rotating presidency of the elite global club.
While Greece is itself not a member of the G20, intense talks were expected on its debt crisis ahead of an emergency meeting of eurozone finance ministers on Wednesday, amid fears it could leave the euro.
French Finance Minister Michel Sapin said that a financing solution needed to be found for Greece but added: "We cannot simply say, we'll finance, we'll finance" Athens without "respect for European rules" in exchange.
Turkish Deputy Prime Minister Ali Babacan, the government's pointman on the economy who is hosting the meeting, expressed hope that Greece, the EU and all its creditors would come up with "mutually accepted results".
"We hope that rationality wins at the end," he said.
New leftist Greece Prime Minister Alexis Tsipras vowed he would be "unshakeable" in implementing his anti-austerity election promises in comments sure to annoy Germany, whose Finance Minister Wolfgang Schaeuble was also expected in Istanbul.
The OECD, in a new report published in Istanbul, said that "determined and systemic" action to implement reforms were needed to help boost weak global demand and restore healthy growth.
The report said that potential growth had been revised down in most advanced economies and some countries were risking a period of "protracted stagnation".
In a joint opinion piece for the Wall Street Journal, US Treasury Secretary Jacob Lew and British Finance Minister George Osborne said the Istanbul meeting was taking place against a "backdrop of challenges".
