1. Best open source alternatives to Office 365: LibreOffice
The leading free office suite was first developed by nonprofit free software advocate The Document Foundation in 2010 as an offshoot of the now defunct OpenOffice.org.
It supports all major office suites and is compatible with Windows, macOS and Linux, for which it's normally the default productivity suite.
LibreOffice encompasses a full range of office functions covering word processing, spreadsheets, presentations, calculations, diagrams and databases
It's known as the most actively developed open source productivity suite and is both cost and ad-free.