Opposition of a trademark is available when similar or conflicting mark is published in the Trademark Journal, in which a party seeks to prevent a pending application for a trademark from being granted registration. Opposition can be filed only to a pending application. Rectification can be filed for registered trademarks.
Opposing the trademark will prevent the other party from registering the Trademark. The outcome of opposition proceeding is the final decision of the Trademark Registrar
The Decision whether to allow or dismiss the opposition procedings is made by the Trademark Registrar
Opposition grounds vary from case to case, Mostly it will be similar or conflicting trademarks or trademarks which has been filed in bad faith.
You have pay hearing fees incase if the matter is posted before the Registrar.