Observational assessment of decision-making in applied teaching settings usually relies on outcome counts, aggregated performance totals or global rater impressions, without establishing whether a defensible option was available to the learner and actually selected. This paper proposes an event-based behavioural coding framework with four linked components: Opportunity, Choice, Execution and Recovery (OCER). It specifies what counts as an eligible decision event, scores option selection separately from motor execution, and treats recovery as an independent continuation behaviour rather than a residual category. The framework draws on game-centred pedagogy, expertise research and the Game Performance Assessment Instrument, and acknowledges a newer tennis-specific tactical instrument. Its contribution is a diagnostic bridge between a game-based teaching episode and an auditable observational assessment, not a new theoretical claim for Teaching Games for Understanding. The paper presents five tactical problem classes, conditioned-practice examples, a draft event codebook and rater-training rules. Two hypothetical episodes illustrate why a situationally appropriate response can carry educational value even when the point is lost. No empirical dataset, observation frequencies or causal estimates are reported. The framework generates testable predictions concerning learning transfer, inter-rater agreement, and whether opportunity-based indices add explanatory value beyond outcome scores and generic skill tests in university sport instruction.