Can someone oppose my patent?
In some countries, the grant of a patent may be opposed by filling a pre-grant opposition.
If the opposition is successful then the patent may not be granted or the scope of the claims may be narrowed from what was requested. In other countries, the grant of a patent may be opposed by filing a post-grant opposition.
If the opposition is successful then the granted patent may be invalidated or the scope of the claims may be narrowed from what was granted. In some countries, both options may be possible.
This means at least that a granted patent can be invalidated by a third party, if the described invention in the patent was proveable state of the art, at the time subject patent was granted.