Spin chain oxides with different incommensurate hexagonal composite structures have been obtained for a same cationic composition according to the formula Sr3CaMn2CoO9+δ by varying the thermal treatments in air. The cell parameters of these oxides, a∼9.5 Å, c1∼2.5 Å and c2∼3.8 Å, decrease as the deviation from commensurability increases, i.e. as γ = c1/c2 decreases and falls away from 2/3. All the investigated samples exhibit a signature typical of single-ion magnet (SIM), as well as a competition between single-chain magnet (SCM) and short-range ordering (SRO) responses. The balance between the latter two features turns out to be clearly affected by the thermal treatment. The sample quenched at 1450 °C, which structure is almost commensurate (γ = 0.6659, close to 2/3), exhibits a SCM behavior without sizeable indication of SRO, whereas, for the samples annealed at 1300 °C or furnace cooled from 1400 °C with an incommensurate structure (γ = 0.6622–0.6631), the SRO character is strongly reinforced. These changes in the magnetic response are assumed to originate from incorporation of extra oxygen into the commensurate stoichiometric Sr3CaMn2CoO9 oxide, leading to the replacement of some trimeric units built up of two octahedra and one trigonal prism (Oh2Tp) by tetrameric units (Oh3Tp) containing one additional octahedron, with a concomitant appearance of cationic vacancies in the trigonal prismatic sites.