Is OOo (LO) open source?

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
http://www.documentfoundation.org/foundation/

See also: Are libre office and open office the same company?