Primary fonts and weights
The two fonts serve different purposes. Montserrat Bold should be primarily used for headings; Open Sans Regular should mainly be used for body copy. Both font families contain several acceptable weights which are Light, Regular, Medium and Bold. While additional weights are available within the font families, these are the only four that should be used unless a specific case otherwise demands.
Montserrat Bold
The quick brown fox jumps over the lazy dog.
Open Sans Light
The quick brown fox jumps over the lazy dog.
Open Sans Regular
The quick brown fox jumps over the lazy dog.
Open Sans Medium
The quick brown fox jumps over the lazy dog.
Open Sans bold