OSS stands for Open Source Software, refers to software whose source code is available to customers and it can be modified and redistributed without any limitation.
An OSS may come free of cost or with a payment of nominal charges that its developers may charge in the name of development, support of software.
FLOSS stands for Free Libre and Open Source Software or Free Livre and Open Source Software, refer to a software which is both free software as well as open source software.
Here the words:
Both means freedom.