Before you start OPOS method of cooking, make sure your pressure cooker is standardized. This is done so that it works the way it is supposed to, in order for the timings in the recipes to work. OPOS follows precise instructions and it becomes necessary to understand your cooker as the first step.
Heat 1/4C (60ml) water in your 2L cooker on high so that you get the first whistle between 1 and 2 minutes.
Please refer to the video on how to do it.
I own different sized cookers and I am sure you do too. So, how do we go about using them for OPOS?
1. 3 Liter:
You will mostly find OPOSers using 2L and 3L interchangeably for recipes developed for 2L cookers. The capacity of a 3L is approx 1.5 times of 2L pressure cooker and while recipes can be scaled up by a small margin, there is no standardization unfortunately in place for 3L cookers. Depending on the recipes, I tend to err on the side of caution and add minimum of 3-4 tbsp of water along with 1-2tbsp of fat/oil in the base and use the recipe accordingly. Try a simple recipe for the first time around with the scaled up recipe adding a little more water in the bottom (for flash cooking or any recipe that doesnt use much water) and based on the result, scale back on the water next time around.
2. 4 Liter and 4.5 litre:
For using 4L and 4.5L, repeat the standardization technique for 2L cooker but with 1/2C (120ml) water with long whistles between 3-4 min mark.
OPOS has not yet developed recipes exclusively for 3L or 4L cookers and keep in mind that they need NOT work the same way as a 2L cooker. Also, 4.5L cookers are predominantly used in OPOS for starches, dals, any legume, thokku (pickles) and jam type of recipes that rely naturally on the inclusion of some form of liquid.