From my experience, if a site wasn't built in modular way, then it's hard to implement OOCSS without changing existing code.
Looking at your CSS briefly, it seems like there are a lot pages/sections specific styling. One of the main OOCSS principal to separate container and content
. I think the first thing to do is to analyse your site and work out the design styles (commonly used colours, font sizes for headings, list styles etc) and components (navigation, dropdown, meda block). If you have a lot of different styles (e.g. > 6-7 different font sizes and colours, many different list styles), then it might be a good idea to look at the design of your site and see if it's possible to cut down on the less used styles. This will help you cut down on CSS and avoid having to write section specific styles.
So you can take a look to get some ideas how SASS is used with OOCSS.
Hope this helps!