Integrate blog into main site

Small question – i created a WP site, and I want to be able to integrate it into the previous site (it was “designed” to carry the same look). Anyway, people are currently driven to the blog via a link on the navigation from the main site, but it opens a new window.

We can’t figure out how to use the navigation on the main site in the WP site. https://www.osara.org is the main site and https://www.osara.org/journal is the blog.

Thanks!