A patent opposition is a challenge to a granted patent, and is usually launched by a competitor company that is also an active innovator in the relevant technical sector.  Oppositions may be made on a number of grounds: that the invention was not patentable in the first place, e.g. because it had been done before, or that it was an obvious development, or because the invention was not described sufficiently thoroughly that a third party could carry it out, or that the patent is broader than is justified by what the application described when it was filed.

Time limits usually apply.  For example, an opposition made under the provisions of the European Patent Convention (EPC) must be filed at the European Patent Office (EPO) within 9 months of the date of grant of a European patent.  That’s why many companies maintain a patent watch to make sure they become aware of potentially competing patents.

If you think that a patent application by a competitor company ought not to be granted, perhaps because you have already been working on the same developments, you ought to discuss the matter with a patent attorney as soon as possible.

While oppositions may sometimes be resolved purely by arguments in writing, most are resolved at hearings where patent attorneys for both sides make their arguments. Many UK patent attorneys are quite used to flying to Munich, for example, to represent their clients’ interests before the EPO.