You are right, it'd be a good idea to migrate some posts here, but with community wiki and following the attribution guidelines! We aren't just going to copy all the stuff from there and paste it without attribution.
An interesting idea, would be to search in https://web.archive.org for https://www.or-exchange.org, but... I have ninja'd you. Here are all the snapshots. You can select the year from the timeline.
The newest snapshot is from 20 November 2018, but with server errors. There's one with redeirects from 8 November 2018 if you are interested. However, none of the questions has been snapshotted.
However, I didn't even know that such site existed. What were its maintainers? And why was it shut down? Is this information known? Maybe contacting the maintainers of it would help.
The OR-exchange site is now read-only! Enjoy.