Just for general information, the OnyX 2.1.3.dmg does not have an uninstaller. If memory serves, some versions previous to OnyX 2.x.x did have it but not all. I recall that the earlier days of OnyX did not have an installer but the app could be installed by simply dragging it into /Applications.
Like you, I keep .dmg files around in case I need them again and that has come in handy. I try to eliminate old ones from time to time in order to clear out some space.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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