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 (or two-finger-click/tap) 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 not have (correct) results of those calculations, but at least you will have something to work with in another spreadsheet application.