This paper focuses on four strategies of onset reduction employed by a single child (4;0–4;4) acquiring Polish: deletion, coalescence, metathesis, and gemination. Deletion and coalescence occur in word-initial onsets while metathesis and gemination are restricted to word-medial position. The data, which constitute an intriguing ‘conspiracy’ case (Kisseberth, 1970), are analysed within OPTIMALITY THEORY (henceforth, OT; Prince & Smolensky, 1993/2004; McCarthy & Prince, 1995) in which all surface-true ‘processes’ are motivated through the interaction of ranked and violable constraints. The OT account makes it possible to envisage the four strategies as different surface responses to the undominated *COMPLEXOnset which militates against onset clusters. The choice of a particular strategy as well as its restriction to a particular word position is not random but follows from the interplay between *COMPLEXOnset, sonority-based syllable structure constraints (Margin Hierarchy, CONTACT LAW), context-sensitive markedness constraints (CODA CONDITION, *Nasal–Fricative) and faithfulness constraints. The present study confirms previous sonority-based findings, supplies further evidence for universal sonority mechanisms from word-medial clusters, and points to the coexistence of child-specific and abstract adult-based phonological strategies in the child’s system.