零天.

Unfortunately there is no grace period for a termination under the O visa.

Here is the link to the CIS website giving that policy. I believe it is #3.

http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=67cd9369e6367210VgnVCM100000082ca60aRCRD&vgnextchannel=b9ab9ddf801b3210VgnVCM100000b92ca60aRCRD