OG is just practice problems--it won't do you much good on learning
how to work the problems, or on teaching the formulas/equations/processes you need to use to get through things quickly. There are strict processes you should follow even for CR and RC questions, it's not just a matter of "read it and pick out the one you think is right." You've gotta know how to analyze arguments, identify flawed premises and assumptions, find the relevant information for what the question is actually asking, etc.
MGMAT (or any number of other books) will teach you all that stuff. Doing nothing but OG will only continue to reinforce bad habits without teaching you how to stop making those same mistakes.
With you not being a native English speaker, you'll get a lot of value out of simply reading as much English as you can...magazines, newspapers, literature, whatever. That will help with your RC skills and give you a feel for what "sounds" right in terms of grammar, a big advantage for SC.
Very true! OG almost just gives solutions. Thanks a lot! I will now buy some new books and start it all over again!