what's the meaning of "average OECD effort" here?

GNI is "gross national income". It is equal to the total value of what a country produces, plus the net income from overseas investments.

The OECD is an organisation consisting of a few dozen wealthy countries and describes itself as a group of countries who are pursuing "democracy and free markets".

So "average OECD effort" means that the average country from the OECD gives 0.45% of its gross national income to help other countries develop.
(Australia gives 0.3%. The United Nations asks us all to give 0.7%.)

Thanks！