Every time I closed Oblivion I had to run the task manager, because the program would quit responding. Since it didn't really affect gameplay or saves, I overlooked it, and made no real attempt to fix the problem. But, gradually, I began to get an increase of ctd's during the game at unpredictable times.
I am running Vista 32 bit. I had installed OBSE, cobl, OOO, Wrye leveling (great mod), Bag of Holding, and SI. I thought I'd try Super Hotkeys, though I was reluctant to add it, for fear that it would make Oblivion even more unstable. What the heck! I threw caution to the winds, and ever since installing Super Hotkeys, Oblivion is running perfectly: no more ctd's, and when I quit the program it closes without a hitch.
I have no idea why or how it fixed the problem but I am grateful.

Check in the mods forum for Enhanced Hotkeys. You might wanna upgrade.