Hi Lalit,
Do you man you want to integrate the multiple-forests environment with Office 365 online AD? If that is the situation, I feel sorry that this is not a Microsoft supported scenario. A single on-premises Active Directory forest is supported for DirSync.
As far as I know, to achieve this specific requirement (multiple forests), you might need to involve Microsoft partner or senior consultant for a delicate support. You can find the partner related support via this link:
office365.pinpoint.microsoft.com/.../home
Thanks,
Young