Delta oscillations contribute to the human P300 event-related potential evoked by oddball targets, although it is unclear whether they index contextual novelty (event oddballness, novelty P3, nP3), or target-related processes (event targetness, target P3b). To examine this question, the electroencephalogram (EEG) was recorded during a cued task-switching version of the Wisconsin card-sorting test. Each target card was announced by a tone cueing either to switch or repeat the task. Novel sound distracters were interspersed among trials. Time-frequency EEG analyses revealed bursts of delta (2–4 Hz) power associated with enhanced nP3 amplitudes to both task-switch cues and novel distracters—but no association with target P3b. These findings indicate that the P300-delta response indexes contextual novelty regardless of whether novelty emanates from endogenous (new task rules) or exogenous (novel distracters) sources of information.