The default extension in LO is .odt (ODF document format if you use Save As - Linux version on the machine I'm working on).
If the document opens in OO it sounds as though the default for .odf is still OO not LO (I don't know how these things work on the Mac, so I can't offer any further help in changing the default open program). In the file manager on Linux (and Windows IIRC - I'm not using that so much these days) the right click context menu Open With option for a file has an Other Application that lets you select a program not already in the list of alternatives to the default; I would have thought the Mac had a similar option
If you use the File menu in LO it should just open a .odf file if you select it.