Building a small room below grade. Walls/Floor will be grouted stacked stone (apx. Rip-rap size, 2"-8" diameters). Roof design is intended to be set and grouted in on top of stone wall. Soil will be loosely placed on top at 1'-0". Assuming 1'-6" soil depth max for any soil resettling. Imgur album link is included below to see added detail.
Remainder of room design accounts for: Fireproofing, waterproofing, drainage, ventilation.
I don't understand the reason for the blocking at the top of the roof 2x6 attachment to the 4x4. That angle change to flat will be a mess.
Presume 150lb per cubic foot o material on the roof. That's a 225lb per square foot dead load. A hallway on a multi story office building has about a 250lb load rating, that is because when hundreds of people jam the hallway in a fire or emergency situation that's what it takes to prevent a collapse. The osb "could" be made to work if you use steel fasteners and high quality glue, if you double it and glue it together before install it would be better. The 1/2 of bearing area of the osb on the roof 2x6 would only work if that was epoxied. Steel bracket and run a steel cable around the mid point and base, tighten that to carry the massive lateral deflection on the roof studs from the "bowing" osb decking. The walls you start with 60lb per foot lateral pressure, studs across the floor to prevent the sides kicking in will be necessary since only 6 feet is 360lb lateral load per sf. Lastly this better have a complete rap off a single piece of EPDM or a similar waterproofing membrane. Any water on that osb and you have a collapse. The initial soil load of 150lb is higher that soil load but you need to estimate water for a rain event. Oh and steel brackets at the top and bottoms of the roof studs. If it's temporary use double the stud stacking and go for it. Wet environment presume you get a summer of use. GET OUT IF THE WOOD EVER GETS WET!!!!! DEPENDING ON THE TYPE OF SOIL AND WATER THAT IS A NO NOTICE INSTANT COLLAPSE AND SOMEONE STANDING UNDER THE OPEN WILL DIE ALONG WITH EVERYONE ELSE IN THAT. Look up residential live and dead load requirement in the IRC since your using stick lumber, there are charts telling you what you need. Good and have fun.