Until when can I stay in US if my visa application is rejected?

[forestcat 0]

Now I am applying to US O1 visa.

My situation is;

- I have been F1 visa holder from June 2011. I graduated college in May 2015 and I started my OPT in June 2015, then OPT ended June 2016.

- In my grace period I submitted an application for O1 visa, and USCIS sent me RFE (request for additional evidence).

The due date for submitting additional evidence is the end of Oct 2016.

My question is;

①If the final decision of USCIS will be `rejection` to my O1 visa application, will I be seemed as an overstayer right after I get the decision?

Or will USCIS give me some days to leave US?

②Will it be possible to appeal against the rejection? If I appeal, will I be able to stay in US during my appeal is under dispute?