Also most ovens fluctuate greatly. If your oven is set to 148 it's probably not going to stay there consistently. It's going to like drop into the 130s then over heat to the 150s.
I'm thread bumping, but I'm sure other newbie's will come across this like I did so I thought I would provide some useful info on this matter.
Now.....regarding mashing in the oven.
Thermodynamics 101
Q=UAdeltaT
Q would be the amount of heat transfered either into or out of your mash.
U is the heat transfer coeff. of air, which air is really really bad transfering heat (this is good for us)
A is the area, or the surface area of your pot. I'm guessing most pots are made with a minimal amount of material respective to their volume because they could sell them cheaper. This makes A as low as practical.
deltaT is the difference in your mash temp and the difference in the oven temp which we intentionally keep as low as possible.
U is always going to be small with air, A is as small as practical, and so is delta T. small number * small number * small number = really small number
Have you ever set a mash pot outside, even when it's really cold, and it still takes forever to cool? That's because air is so poor at transfering heat. Even if your oven is 10 or 20 degrees off from your mash temp, it will take quite a long time for your mash to change even 1 degree.