this is what I need
FReepers are the best!
I have used OpenOffice.org and I like it. Unlike you, my use case is primarily Microsoft Word, not Excel - but only because my home use does not involve much number crunching.
This is my own experience, which may differ from what others have experienced: it reads and writes foreign document formats just fine for the most part.
On rare occasion, OpenOffice.org may save an MS Office Word document in such a way where it can be viewed but the formatting will be just a little different when viewed once again in Microsoft world.
On rare occasion, OpenOffice.org may save an MS Office document in such a way that visually everything looks fine, but certain pages won’t print in Microsoft world.
However this was observed over many years of use and the latest versions may not even have these behaviors any more.
YMMV
> this is what I need
I use LibreOffice, very compatible with MS/365 Office for all the usual stuff (Word, Excel, etc.), but you must be careful to tell it to write newly created files in MS/365 Office format (.docx, .xlsx) because its defaults are not the MS ones.
Other than that reminder, it's pretty much seamless.