A search is presented for supersymmetric partners of the top quark (top squarks) in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from Formula omittedquarks, and missing transverse momentum. The search uses data from proton-proton collisions at Formula omitted collected with the CMS detector, corresponding to an integrated luminosity of 137 Formula omitted. Hypothetical signal events are efficiently separated from the dominant top quark pair production background with requirements on the significance of the missing transverse momentum and on transverse mass variables. No significant deviation is observed from the expected background. Exclusion limits are set in the context of simplified supersymmetric models with pair-produced lightest top squarks. For top squarks decaying exclusively to a top quark and a lightest neutralino, lower limits are placed at Formula omitted confidence level on the masses of the top squark and the neutralino up to 925 and 450 Formula omitted, respectively. If the decay proceeds via an intermediate chargino, the corresponding lower limits on the mass of the lightest top squark are set up to 850 Formula omitted for neutralino masses below 420 Formula omitted. For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1.4 Formula omitted and 900 Formula omitted respectively for the top squark and the lightest neutralino.