Hi and welcome to the forum!
Better avoid the OOXML format, have a look at: MS Office 2007 OOXML file format (docx, xslx, pptx, ppsx)
Standards like ODF (native format for OOo) have been designed for interoperability.
NB: LibreOffice has decided to support OOXML. Not the best decision IMHO. OOo could save in OOXML but it was the team's decision not to allow that.