An opposition may in practice be based on one or more of the following grounds:
The invention is not novel or inventive
This ground is the most commonly used ground. If an invention is the same as, or obviously follows from, a product or method that was available publicly before the filing date of the patent, this ground can often be invoked successfully.
Novelty and inventiveness are thoroughly investigated during pre-grant examination, but of course there may be publications that the examiner did not find or did not use. Such publications can be used during the oppostion procedure to argue that the claimed invention did not meet the basis requirements for patentability.
Often the opponent, being active in the same technical field, has better knowledge of what was made public and has access to different documents than the examiner. Typically this concerns brochures, trade magazines, symposium proceedings, exhibitions, and standardisation committees. The examiners at EPO rarely use those kinds of disclosures spontaneously, because they are not so easy to find.
If the opponent does not have evidence of such public disclosures, a specific prior art search is strongly advised. Article 99 can perform this for you.
The invention is not sufficiently disclosed
If a skilled person in the particular technical field can’t reproduce the invention without undue burden, based on his knowledge and the disclosure in the patent, the patent can be revoked in opposition. During pre-grant examination this aspect is usually not investigated in depth by the examiners, and the benefit of the doubt is often given to the applicant. This ground of opposition, if presented well, offers good prospects of success to an opponent in some cases.
Extension of subject matter
If the features as claimed were not disclosed in the application as filed, the application may not be granted. This is not uncommon, especially if the claims as granted are amended compared to the application as filed. This argument is regularly usuable.
It is especially attractive because if it is successful, it is very difficult, and in many cases impossible, for the patent proprietor to rescue part of his patent by making amendments. Often, if it can be shown that an error, even a small error, occured in allowing an amendment during pre-grant examination, such an error is fatal to the maintenance of the patent.
Exclusion from patentability
This means that by its nature the invention should not be patentable: Eg: Not industrially applicable, not technical, medical treatments, scientific theories. This ground is not used very often.