This is probably hard to say there never really is a certain time you should use OOP
But think of OOP as in the tutorials you've probably seen as objects,
Each Object within the class has some sort of function to the entire outcome of the class
So a good use would be say a dbclass
In a dbclass you'de have
A Connection
A place to select the database
A Query bit
Something to fetch data from the query
etc..
Thats a good example on when to use one, as it is Always Used throughout a site, Has Many parts(objects0 to the overall result. And is faster than using a whole pile of functions.