About the difference in these sentences with のを and のが


I've been struggling to find the difference between those two structures, is
one of them wrong or they are both right and have a difference in
meaning/emphasis

> 俺が走るのが見た

> 俺が走るのを見た




の is a nominalizer, が is a subject marker, を is an object marker, and 見る is a
plain transitive verb meaning "to see". 俺が走るのが見た is plain wrong because the
act of running cannot see something.

  * ❌ 俺が走るの **が** 見た。(incorrect)  
❌ My running saw [something].

  * 俺が走るの **を** 見た。(correct)  
[Someone] saw me running.

However, there is another verb 見える which means "to be visible", "to be in
sight", "can be seen". This can take an action as a subject. See: [What is the
difference between 見る and
見える?](https://japanese.stackexchange.com/q/39242/5010)

  * 俺が走るの **が** 見えた。(correct)  
[People] could see me running. / My run was visible.

