For standard Oreo cookies, how many would be a kezayis by volume? I have heard maybe 2.5 or maybe 3.
According to the site projectsbyjen.org, the dimensions of an Oreo are 1.75 inches in width, and .314 inches in height. Since there's, surprisingly, not a lot of easy to find info on Oreo dimensions, let's go with this. A k'zayis (according to berachot.org, quoting Rav Bodner's "Halachos of K'zayis") is 28.8 cm³ (the most stringent view (I think), equal to half the volume of an egg.)
So! Approximately 2.3 cookies in a k'zayis, though because of empty space under the ridge, and the grooves in the cookie's design, maybe move it a little closer to 3.
Looks like you heard right. :)