To actually open an ODS file with all the formulas you need to install Open Office or Libre Office or some other program that understands ODS format, but if you just need to quickly get the data and the basic structure of an old file:
- Right-click the ODS file and select Open With -> Other
- Select TextEdit app
- Select all of the content
- Paste it into a new Numbers spreadsheet
If you had formulas in some cells, they will now have (correct) results of those calculations, but in my case that was all I really needed.
What I wonder now is - if TextEdit can read ODS, why can't Numbers do it directly?