EPO Opposition Strategy: Building a Winning Prior Art Portfolio

EPO oppositions are heavily front-loaded: the prior art you file with the opposition notice is largely what you have to work with throughout the proceedings. Building a prior art portfolio that can survive methodical patentee responses — and withstand oral proceedings — requires a more disciplined approach than simply searching for anything that looks relevant.

The Opposition Timeline and Its Strategic Implications

The EPO opposition window is nine months from the date of grant publication (Art. 99(1) EPC). This deadline is absolute — no late opposition is permitted, and no extension is available. Within this nine-month window, the opposition notice must be filed and must contain all substantive grounds and evidence that the opponent wishes to rely on.

While additional evidence may be submitted after the nine-month deadline in limited circumstances, the EPO's admissibility rules under Rule 116 EPC and the Guidelines for Examination mean that late-filed documents face discretionary exclusion. The practical consequence: treat the opposition notice as a complete document. Do not plan to supplement it later.

Selecting the Right Grounds

European oppositions can be filed on three grounds under Art. 100 EPC: lack of novelty (Art. 54 EPC), lack of inventive step (Art. 56 EPC), and insufficient disclosure (Art. 83 EPC). A well-constructed opposition typically leads with Art. 56 rather than Art. 54, for reasons that experienced practitioners understand intuitively.

Novelty attacks are binary: either the prior art discloses every element of the claim, or it does not. If the patentee identifies a single claim element that is not directly and unambiguously disclosed in a single reference, the novelty attack fails entirely. Art. 56 inventive step arguments are more resilient — they accommodate combinations of references, allow for technical reasoning about what the skilled person would have been motivated to do, and provide more vectors for argument when the opponent's primary reference is not perfect.

Tactical Principle: Use Art. 54 to Set Up Art. 56

Even when the novelty attack is unlikely to succeed alone, a well-crafted Art. 54 argument establishes the claim elements present in D1 (the closest prior art) — which then directly feeds the Art. 56 analysis by defining what the differences are. Use the novelty analysis to frame the inventive step argument, not as a standalone attack.

Mastering the Problem-Solution Approach

The EPO's Problem-Solution Approach (PSA) is not optional — it is the obligatory analytical framework for Art. 56 inventive step under EPC examination and opposition. Understanding how to use it offensively is central to opposition strategy.

The Problem-Solution Approach — Four Steps

Identify the Closest Prior Art (D1)

Select the single reference that represents the most promising starting point for the skilled person — the one most similar to the claimed invention in structure, function, and technical field. The choice of D1 drives everything that follows; select it to maximize the perceived obviousness of the differences.

Determine the Distinguishing Features

Identify the features of the claim that are not disclosed in D1. Be precise — the patentee will argue over this list, and imprecise identification of differences can undermine the subsequent technical problem formulation.

Formulate the Objective Technical Problem

The objective technical problem is defined by the technical effect achieved by the distinguishing features — not the subjective problem stated in the patent. Formulate it at a level of generality that a skilled person would naturally address with multiple technical approaches, making the claimed solution more obviously one of several reasonable choices.

Assess Obviousness Using D2 (and Common General Knowledge)

Show that the skilled person, starting from D1 and faced with the objective technical problem, would have found the claimed solution obvious — either from D2 alone, or from D2 combined with common general knowledge (CgK). The key discipline: the motivation to combine must be evidenced, not assumed.

Building the Prior Art Portfolio

The quality of an EPO opposition depends almost entirely on the quality of the prior art search. The nine-month window is often spent predominantly on search, with brief preparation. This prioritization is correct — a mediocre brief built around excellent prior art will outperform an excellent brief built around weak references.

Search Sources Beyond the Obvious

The obvious search databases — Espacenet, Derwent, PatSnap — are necessary but not sufficient. Prior art that does not appear in patent databases is often more persuasive to EPO Opposition Divisions because it is harder for patentees to distinguish. Productive non-patent literature (NPL) sources include:

Academic conference proceedings (IEEE, ACM, ACS, Nature-family journals) — often contains earlier disclosure than corresponding patents
Industry white papers and technical standards (ISO, IEC, IEEE standards documents)
PhD theses in the relevant technical field — frequently represent detailed prior disclosure of specific technical approaches
Product manuals and technical specifications predating the priority date
Sales catalogues and trade show materials — establishing commercial availability can support both prior art and common general knowledge arguments

Date Authentication

Every item of prior art must include evidence of its publication date before the priority date of the patent under attack. For patent documents, this is automatic. For NPL, the date evidence must be explicit — title page with publication year, publisher's copyright notice, or archive metadata. EPO Opposition Divisions are strict: undated or ambiguously dated documents will typically be excluded or given minimal procedural weight.

Oral Proceedings Strategy

EPO oppositions that are not settled or decided on the papers proceed to oral proceedings — a single hearing day before a three-member Opposition Division. The hearing is typically scheduled 12–24 months after the opposition notice. Several strategic considerations apply:

File Substantive Observations Before the Oral Proceedings Communication

The EPO typically issues a preliminary opinion (a "Communication under R. 84(2) EPC" or a summons communication) before oral proceedings, indicating the preliminary view of the Division. File a substantive response to this communication even if your position appears favorable — failure to respond can be read as de facto acceptance of the Division's framing of the issues.

Prepare for Claim Amendments Under Rule 80

Patentees routinely file auxiliary requests — amended claim sets — in response to opposition. An opponent who has not anticipated the range of possible amendments will be unable to respond coherently during oral proceedings. Before oral proceedings, map out the claim landscape: which elements could the patentee narrow and still maintain commercial value? For each plausible auxiliary request, prepare a corresponding inventive step argument.

An opponent who walks into oral proceedings with a single attack against the main request will lose when the patentee's first auxiliary request is allowed. Prepare for at least three contingency positions before the hearing date.

Conclusion

EPO opposition is among the most technically demanding proceedings in global patent practice. The PSA demands precision in problem formulation that most practitioners underestimate. The front-loaded evidence rules require a level of prior art search investment that many clients balk at — until they understand that thin prior art filed with the opposition notice cannot be supplemented without court approval. Build the opposition notice as if it is your only submission. It effectively is.