Please, is OpenOffice (LibreOffice) open source and why?
Both Office suites are Open Source.
OpenOffice derives from Star Office (http://en.wikipedia.org/wiki/StarOffice) which was acquired by Sun Microsystems, who later released the source.
LibreOffice derived from OpenOffice when Oracle took over the project (http://en.wikipedia.org/wiki/LibreOffice)
I can’t answer the Why part…
You can read the Manifesto and Values of the The Document Foundation, the organization behind LibreOffice here