Which iMac? If you trust OCLP, then you can often get onto the latest patched OS, but not always without issue. Check on their website for more details.
I'm using it on a mid-2014 MBP without any major issues. That said, if it was a 'mission critical' machine, I would ditch it for a new one. The new Mac Mini's are excellent value, for example (well, base specs).