Opponent and representation
An opposition against a granted European patent can be filed by anyone. No interest needs to be demonstrated.
Parties not having a residence in the EPC member states need to be represented by a European Patent Attorney. There is no such requirement for parties having a residence in one of those member states. However, it is very strongly recommended also for those parties to be represented.
An opposition is an inter partes procedure. This means in essence that the patent proprietor and the opponent(s) are parties to the proceedings, and an Oppostion Division (OD) of the EPO acts as impartial arbiter. Whereas during examination the examiners may be forthcoming to the applicant and make suggestion to expediate grant, the people in the OD need to be strictly impartial.
Importantly, an oppposition needs to be filed within nine months of grant. This is an non-extendible deadline. All the arguments need to be submitted as well within this deadline. Considering that there may be significant preparatory work, you are advised to contact Article 99 as soon as possible, preferably at least 2-3 months before the nine month deadline.
An opposition procedure usually involves two rounds of written arguments and usually ends with oral proceedings. Immediately at the end of the oral proceeding a decision is taken.
The result of an opposition can be that the patent is revoked or upheld. A partial revocation is also possible, and is actually quite common.
The losing party may appeal the outcome of the opposition. This needs to be done within 2 months.
Article 99 will also be happy to represent you during the appeal proceedings.