A B737-800 was dispatched with an engine generator on MEL and an APU with a history of low oil pressure auto shutdowns. While inflight the APU auto shutdown causing the crew to declare an emergency and divert single generator to a nearby airport with suitable weather.

Narrative

I was pilot flying. The aircraft had the left generator placarded inoperative; and the APU was the 'second' generator per the MEL. After departing; approximately halfway to our destination; in cruise; the APU lost oil pressure and automatically shutdown. After consulting the QRH; the Captain declared an emergency as we were now down to one generator; night IMC; wintertime conditions. Checking weather at the nearest airports; we determined that the weather was not suitable for our condition; and we were seeking a VFR; if at least marginally VFR airport. After checking several airports; we decided on an airport we had just passed with 4 miles visibility; 1;000 FT; 11;900 FT runway; braking action good. The Captain did a great job working the QRH and normal checklists; and managing the flight deck as we flew to our divert airport. We discussed the possibility of the remaining generator failing; and then him performing the 'battery only' approach and landing. Fortunately; the remaining generator did not fail; and we landed uneventfully. This flight was my second emergency landing in less than two months. Both emergencies occurred from items that were previous write-ups in the logbook or placarded MEL items of important systems. In the case of this event; my only suggestion is that placarding a generator inoperative; with APU having a prior recent history for shutting down to low oil pressure (or any reason/history in the logbook); especially wintertime; is a bad idea. This was a 'learning experience' for me; and will guide my decision making in the future. This event is a good example of an emergency being entirely preventable.