The act was originally created in 2008, and in the interim time the OECD has worked to update the act. The updated definition is based on the same elements that are in the ODAAA: the main objective is to promote the economic development and welfare of developing countries; and the assistance must be concessional in character.
The main change came about due to negotiations at the OECD and consensus agreement. It relates to a minimum grant element required for assistance to be considered as ODA. This is traditionally applied to concessional sovereign loans to ensure that donors are giving sufficiently generous loan terms to count as ODA.
More precisely, just to be concrete, the old definition had a grant element of 25%, and now the updated definition has a graduated scale going from 45% down to 10%. This encourages more concessional funding to the least developed countries.