Even the most casual fan of old school R&B should get acquainted with the O’Jays, and The Essential O’Jays is a good place to start. This collection offers the highlights of their classic ‘70s hits for Philadelphia International. Like other purveyors of Philly Soul, the trio harmonized impeccably over well-tailored strings and rhythm tracks. What set them apart was the political content of their songs — when lead vocalist Eddie Levert sang about betrayal in “Back Stabbers” or raw need in “For the Love of Money,” he challenged his audience to think as well as groove. Tracks like “Survival” and “Give the People What They Want” are insistent demands for justice amidst the deceptions of the Nixon Era. The flip side to these angry sentiments is offered in “Love Train” and “Put Your Hands Together.” The O’Jays are masterful at balladry as well, melting hearts effortlessly in “Use Ta Be My Girl” and the gospel-rooted “Stairway to Heaven.” Whatever the song, Kenny Gamble and Leon Huff’s production stays crisp and inventive, whether the mood is spiteful (“992 Arguments”) or celebratory (“Message in Our Music”). The Essential O’Jays is just that: mandatory listening for anyone with a taste for soul.