1) Add the libraries that you want to use in your theme's compony.info.yml file:

libraries:
  - compony/global
  - compony/font--open-sans--bold
  - compony/font--open-sans--bold-italic
  - compony/font--open-sans--extra-bold
  - compony/font--open-sans--extra-bold-italic
  - compony/font--open-sans--italic
  - compony/font--open-sans--light
  - compony/font--open-sans--light-italic
  - compony/font--open-sans--regular
  - compony/font--open-sans--semi-bold
  - compony/font--open-sans--semi-bold-italic

(Only include the library definitions of the fonts that you need)

2) Use it in your CSS as follows:

font-family: 'open-sans--bold';

All the fonts are named using the BEM-naming structure.