The website is made with Hugo, a static site generator.
The theme used is hugo DPSG which is a fork of Mainroad Hugo theme.
To learn more about hugo visit its documentation: https://gohugo.io/documentation/
Download hugo from here, here is a video tutorial.
git clone --recurse-submodules https://github.com/osac/osac.github.io
Then use,
go server -D
To open up a localhost server. Go to browser at https://localhost:1313 or the url specified in output to above command.
Go to content/post/ folder. Copy existing post and give it new name. Change the front matter tags to appropirate one and write your post in markdown.
If you need to add images, put them in /static/img/ folder by creating a directory same as post name and placing images inside that directory.
You can view your changes through the go server -D
command as mentioned above.