obtain fresh macos for old iMac?

It seems you have magic fingers.

First, a digression: the dosdude1.com process calls for a flash drive of >= 16 GB, so I eventually found an expendable SanDisk 64 GB - which worked, but very very slowly. Nothing I, Disk Utility, & TechToolPro 14 could do could speed it up, or diagnose it - I suspect mostly because it would time out before the unmount process could start. Yet another weirdity along the way of this problem project.

Anyway: tonight, the 3rd or so time I tried it today, our friend Option-⌘-R finally worked all the way, downloading & installing macOS Catalina without teasing through the EU license screens before going AWOL. (Or so I assume - after the usual OS install displays and delays, I now see a "Welcome" [Pick a country] window, which I will shutdown on and let the purchaser (if any) work through for themselves.) It seems Apple slew the gremlins again - thanks, Tim!

I won't pursue the dosdude1.com mysteries any further tonight, but will add this thread to my troubleshooting archive just in case.

Thanks to all for your know-how, time, and patience!

Thanks for the quick response!

When I did as you suggested, the process went as far as offering to reinstall the MacOS, with only one choice (Mountain Lion (?!?)). When I clicked that, the next window told me to select a disk to install onto - but didn't show the internal HD (or any other, since no other was plugged in). I gave it a while to see if anything would show up, then backed up to the "OS X Utilities" window; tried again, got the same (non)result.

What now?

No, I haven't, because I have had enough problems getting to this point that not being able to boot up from the internal drive would have stymied me along the way. I have duplicated the contents of the internal to an external HD, and started up successfully from that, but I also use (other volumes on) that external HD for other purposes.

Tentative progress report: using your Option-⌘-R suggestion, I got the system to update itself to Catalina - but still under my name.

So, I decided to boot from the external HD and to erase the internal with Disk Utility, and chose the maximum erase option - which I had not tried before, and now know will (at 7 passes, and the first not done yet in over an hour) take all night, if not longer.

Tomorrow (I hope!), I'll try the Option-⌘-R download again - but it looks like you've given me the magic key. Thanks!

The erasure project took until late this morning, and threw me an error message:

Fixing damaged partition map/

The operation couldn’t be completed (com.apple.DiskManagement error -69874

Operation failed...

So I restarted from the external HD, used Disk Utility>First Aid until it said it was okay, tried Option-⌘-R, couldn't connect. (I should've mentioned this iMac lost its Ethernet port from a lightning strike; somewhere since my 1st message, it also forgot my wifi log-in...) Short version - I got connected, now the reinstall command brings up the Mojave window, but the "click Continue" button produces only a long pause, then a "The recovery server could not be contacted. (OK)" message (three times so far).

Perhaps my gremlins jumped through the wifi link and now romp across the servers in Cupertino. All I can think of at present is taking a break and seeing if the recovery system will work later. <sigh>

The "reinstall MacOS" option continues to say it can't connect to the recovery server. The "macOS Utilities" window which returns when I click the OK button for that error message includes a "Get Help Online" item, and that has options (such as Weather Channel) that show my wifi connection continues to work, so I guess that means the problem lies on the Apple end.

Will try the dosdude1.com patches in the stackexchange.com/... link you provided - the others seem to require a comfort level with Terminal I don't have. Due to some unavoidable interruptions ongoing at my end, not sure when I'll report back on same.