It means something that qualifies as awesome, just double.
Between AWESOME and AWESOMENESS, right in the middle.
Can be used as short version of a very long word aaaaawwwwwesome.
2. "When I get sad, I stop being sad and be OOSM instead. True story."
3. a) That club was awesome, but this one is OOSM!
4. It can be used also to double the effect for the cool words like:
amazing, astonishing, inspiring, breathtaking, daunting, formidable, impressive, magnificent, majestic, overwhelming, redoubtable, stunning, stupefying, wonderful, wondrous, mind-blowing, top, fly, spectral, champion, to rule, fabulous, eye-opening, incomprehensible, inconceivable, incredible, unbelievable, unimaginable, unthinkable; extraordinary, phenomenal, rare, sensational, spectacular, unique, notable, noticeable, outstanding, remarkable, impressive, smashing, striking, mind-bending, mind-blowing, mind-boggling, animating, energizing, enlightening, enlivening, exciting, galvanizing, invigorating, stimulating; alluring, attracting, attractive, beguiling, bewitching, captivating, charming, enchanting, entertaining, enthralling, fascinating, interesting, blue-ribbon, like-a-boss, first-class, numero-uno, peachy, tip-top, top of the line, better, slick, nifty, capital...
i.e.: Hardcore beautiful = OOSM