The free OpenOffice suite is relatively compatible with Microsoft Office, but not perfectly. It's best to understand what will work with what.
For simplicity's sake, I'm sticking to word processing files here. But the basic rules apply to other office formats, as well.
When you save a document, OpenOffice defaults to its own .odf file format. Microsoft added support for .odf files with Office 2007 SP2. If someone with whom you're sharing files has a pre-2007 version of Office, or a copy of Office 2007 in bad need of an update, they will not be able to load your file.
Which is why, if you're concerned about sharing documents, you should save them as Microsoft Word .doc files, instead. This is about as universal a word processing format as you can find, and it's unlikely that anyone with a computer will be unable to handle it.
As I write this, OpenOffice cannot create files in the newer Microsoft Office .docx format, although it can read existing files. That's fine. If you're concerned about compatibility, .doc makes a lot more sense.
To change OpenOffice's default format, select Tools, Options. In the right pane, expand the Load/Save section and select General underneath it. You'll find the "Always save as" option at the bottom of the dialog box.