If I completed first year from September 2012 to April 2013... In may; will I be in Level 1 or Level 2 according to OscarPlus?

I, but you can try for II (be clear in your cover letter "i just finished my first year, and will be returning to xx program in September")

Chances are slim either way.

You don't formally register in the next level up until September - this applies for each year. If it's a problem for some (academic) reason, they can usually give you waivers or something. I don't know about OSCARPlus though, it depends why you're asking.