As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED INTO ORD DUE TO LOSS OF GREEN HYDRAULIC SYSTEM FLUID. THE AIRCRAFT LANDED SAFELY AND WAS TOWED TO THE GATE. MAINTENANCE FOUND A DAMAGED O-RING FOR THE GREEN HYDRAULIC SYSTEM POWER TRANSFER UNIT MANIFOLD CAUSING A LEAK. REPLACED THE O-RING AND ACCOMPLISHED A SATISFACTORY LEAK / OPERATIONAL CHECK PER AMM 29-11-15.