B737-800
Flight Crew Last 90 Days 154; Flight Crew Total 8500; Flight Crew Type 4500
Aircraft Equipment Problem Critical
Flight Crew Diverted; General Declared Emergency
Aircraft
In cruise at FL380 we experienced a complete Hydraulic B quantity loss. Looking at Hydraulic Quantity we noticed 0% displayed and the ENG driven B Hyd pump 'Low Press' light was on.  The Captain took the controls and radios as I started the QRH. We declared an emergency and were immediately given direct ORD. We carefully went through each item lost from the B system and checked the runway distance. We asked and quickly received any runway we wanted at ORD. Weather was day VFR; winds 320/7. Once we reached the approach part of the QRH; I took over ATC communications and the Captain talked with the flight attendants and made a PA to the passengers. I also wrote a quick message to Dispatch via ACARS....'Loss of B Hyd sys. Divert to ORD' or words to that effect. Time didn't allow for further communication. Once we checked in with Approach I told ATC again that we had a flight control problem and that an 8NM final would work; knowing that lowering the flaps would take some time. We also asked to have the emergency equipment standing by. We were given Runway 28 which was the longest runway in use. On downwind we continued with the QRH and started lowering the flaps electrically. The whole process went much like the simulator training for this scenario. I also asked ATC for alternate missed approach instructions of straight out to 3;000 feet in consideration of the process it would take to raise the flaps. The overweight Flaps 15 approach and landing was flown by the Captain to touchdown.  We then taxied to the gate with emergency equipment in tow.
After suffering a complete Hydraulic B system failure; the flight crew of a B737NG declared an emergency and diverted to a nearby airport where they conducted an uneventful overweight landing.