Studying different sickle cell genotypes may throw light on the pathogenesis of sickle cell disease (SCD). Here, the clinical profile, red cell sickling and K$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ permeability in 29 SCD patients (15 patients with severe disease and 14 with a milder form) of HbA/S-Oman genotype were analysed. The super sickling nature of this Hb variant was confirmed. The red cell membrane permeability to K$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ was markedly abnormal with elevated activities of P, Gardos channel and KCl cotransporter (KCC). Results were consistent with Ca$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ entry and Mg$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ loss via P stimulating Gardos channel and KCC activities. The abnormal red cell behaviour was similar to that in the commonest genotype of SCD, HbSS, in which the level of mutated Hb is considerably higher. Although activities of all three K$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ transporters also correlated with the level of HbS-Oman, there was no association between transport phenotype and disease severity. The super sickling behaviour of HbS-Oman may obviate the need for solute loss and red cell dehydration to encourage Hb polymerisation, required in other SCD genotypes. Disease severity was reduced by concurrent α thalassaemia, as observed in other SCD genotypes, and represents an obvious genetic marker for prognostic tests of severity in young SCD patients of the HbA/S-Oman genotype.