OPM or Original Philippine Music began its rise during the 70's. During this time the Filipino mainstream music had much foreign influence and the music was more dance oriented with the world evolving and changing with the times.
Pop groups then showed the emergence of VST and Co, Tito Vic and Joey, Hotdog, and APO Hiking Society to name a few. Although their music was called OPM, there was a striking resemblance with their sound to the Pop music playing in the West.
Although the sound was similar, their music is considered the music of the times because they portrayed their sound on the context of their experience as Filipinos and not on what was going on from else where. They they kept in within their home turf and were able to develop the Manila Sound that even until today still reverberates around the city.
Jumping into the present day, most of the current popular musical influences still are beyond Philippine borders, through KPop (or Korean Pop). Compared to their pop music predecessors however, everything from the look, sound, even the feel of the music and songs are totally out of context of being OPM.
True OPM tells us a story, a story that is unique and showcases how life is lived through the beats in the music. Then again, maybe I am mistaken, and this Ppop thing now is what we get to call as the "Manila Sound".