What model/year iMac do you have, and what specs does it have? That will determine whether you can run OCLP, and by extension modern macOS or not.
If you have less than 8GB of RAM and a mechanical hard drive, forget about OCLP. Having at least 8GB of RAM, and most importantly an SSD is an unofficial requirement for running a modern version of macOS.
You can run macOS on less than 8GB (I used OCLP a couple years back with 4GB of RAM on Big Sur), however it is not a pleasant experience at all if you have more than one or two browser tabs and/or several apps open at the same time. Since my experiment with 4GB was not out of any necessity for a modern machine, but rather to just play around with once every so often, 4GB was OK-ish.
An SSD on the other hand is an absolute (unofficial) requirement. The macOS file system, APFS, has only been optimised for SSD's. Performance on even a "fast" mechanical drive is abysmal. I'd go as far as saying all modern OS's (Windows 10/11, macOS and heavier modern Linux distros) need an SSD to be usable.
Sidenote: A Fusion Drive is a mechanical hard drive. If you have a Fusion Drive, ditch it for an SSD if you want to run OCLP.
