You have to report the company you are working for, to your DSO to update your i20. If they are asswipes they will terminate your status because you donot meet the legal requirements for the OPT. If you have a business degree or computer science degree, it might be easy for you to convince them; but if you have a nurshing degree and you find a job as a manager in a department store (ahhm convenience store), it might be a hard sell. 
Some schools require appointment letter, some will just suffice with your email stating thtt you are interning or working in a X company. So fire up an emial to your DSO and ask what they need.