The new title, which will cover all aspects of popular music, joins the Sunday broadsheet's other sector-specific titles, Observer Sport Monthly and Observer Food Monthly.
OMM is to be edited by Caspar Llewellyn Smith, who will leave his current position as assistant editor of the Telegraph Magazine in July.
The former editor of The Daily Telegraph's Saturday Arts & Books section said he wants OMM to be as wide-ranging as possible. 'People have catholic taste in music. There isn't one single (music) magazine out there at the moment that satisfies me,' he said.
OMM, whose existence was denied by The Observer just a few months ago, is to follow a similar pattern to the sport and food monthlies, Llewellyn Smith said: 'They both speak to people who aren't necessarily that interested in food or who are not sports fans. But people say they keep them round the house and end up reading lots of them. That's what I want to emulate (with OMM).
I want the whole thing to be positive, rather than doom and gloom about the music industry.'
It will not resemble a trade title or any current specialist music magazine, he added.
OSM and OFM generally carry celebrity-based features, although both set much store by quirky pieces and human interest material. Observer editor Roger Alton said: 'OMM will offer an original, vibrant and exciting look at the music world and, like OSM and OFM, will tackle the subject matter in its broadest sense.'
The Observer has been pleased with the performance of its two spin-off products over the last couple of years, and was believed to have several possible monthlies in the pipeline at the start of the year.
OSM, whose editor Matt Tench was replaced by Jason Cowley last month (PRWeek, 9 May), was the first of the titles to be launched, in May 2000.
OFM, which is edited by Nicola Jeal, followed in April 2001.