FIG. 1 is a perspective view of a reverse osmosis cartridge showing the new ornamental design; FIG. 2 is a front view thereof, the front and rear views being a mirror image; FIG. 3 is a left side elevational view thereof, the left and right views being a mirror image; FIG. 4 is a top plan view thereof; and, FIG. 5 is a bottom plan view thereof.