OOHMA was launched last year by rebel ambient media owners, who were frustrated by what they saw as the OAA's failure to address their needs or represent them to the advertising industry, despite the sector receiving a growing share of ad revenue.
The dispute has now been resolved with the creation of a new OAA membership tier - the non-traditional and ambient sector - giving members a new code of practice.
Alan James, the chief executive of the OAA, said: "I am very pleased that we are now one family and I am looking forward to working with the new companies. I believe it is a natural development and one that will be greeted very positively by the media industry."
OOHMA had been beset with problems since it was formed - initially, there was a public dispute over how much the organisation should pay its chairman, the former Postar managing director Richard Holliday, and there were further problems establishing a code of practice.
Jessica Hatfield, the group chief executive of The Media Vehicle and a founding member of the OOHMA, said: "We are in an age where media integration and media neutral planning has started to become part of the innovative agencies' mindset. By coming together, we ensure our original aims of increased accountability, more research and setting an industry standard becomes part of that planning process."