Is LO compatible with Apache OpenOffice on same computer?

Is it possible to run OpenOffice & LibreOffice on same computer (Windows 7 Pro)? I’d like to compare before I switch, but the LO install help recommends removing OpenOffice first because of file affinities. I’d hope if I clicked on an .odx file I’d just get a choice of opening in LO or OO; but I could live with starting the program first & oopening the file within it, for the duration of the test.

Yes, it is compatible. Occasionally there are conflicts over file association but in general you can have the choice to “Open with” both programs.

As you suggest the problem can be easily overcome by starting the program first and opening the document.

Please be aware that OpenOffice Quickstart must not be loaded when you install LibreOffice.

Refer this answer about a Base incompatibility.