Up to nine months after publication of the mention of grant of a European patent, anyone (except the proprietor, who is not allowed to oppose his own patent) may file a notice of opposition to the patent with the EPO. The person filing an opposition is called the opponent.

To search for patents against which an opposition has been filed, enter the code 01 in the Opponent search field. To narrow down your search, combine this code with other search criteria, i.e. a date of publication or classification symbol.

The name format for an opponent who is a natural person consists of the last name followed by the first name. The full name should be entered in inverted commas ("muller stefan") and with the field identifier op in the Smart search screen, e.g. op="muller stefan". The name can also be entered in inverted commas in the Opponent field in the Advanced search screen.

Apostrophes ('), hyphens (-) and slashes (/) are not recognised and should be replaced by blank spaces. Diacritical characters (umlauts and accents) are disregarded by the system, so entering Müller with or without umlauts will produce the same number of results. However, to retrieve all possible search results for the name Müller, we recommend entering the names' various other spellings using the Boolean operator OR, e.g. Muller or Mueller.

Opponent names are not searchable in documents published in other alphabets (Cyrillic, Greek, Japanese, etc.).