To answer your original question, yes, oats do appear to absorb more liquor than other mash grains. You can overcome this by adding slightly more mash liquor.
I suspect you, like many brewers, pay too close attention to recipe numbers and forget to experience brewing using your senses. If you want 1.25 quarts of liquor per pound of grain, learn what that ratio looks like. That way, you can add whatever ingredient you please; you can mash in and not worry about how many quarts of water you added. Moreover, if you find a recipe that calls for 1.4 quarts per pound, you can just tell yourself to make the mash more soupy; if it calls for 1 quart per pound, you can make it more stiff.
I don't even measure my mash liquor. I add my liquor and grain a bit at a time and stir until the consistency looks "right". Of course, I have the entire amount of liquor needed in one place, so if I use X amount in the mash, I know I simply use "the rest" to sparge.