The electrochemical exfoliation of graphite in an aqueous ammonium sulfate solution, in combination with ultrasound treatment, has been studied. It allows obtaining a high yield of mildly oxidized graphene oxide flakes during 15 min. The oxygen atomic content in mildly oxidized graphene oxide can be set by the ammonium sulfate concentration in the range from 20 to 28% (C/O ratio ∼3-5). The minimum of oxidation degree has been found to correspond to the electrolyte concentration of 0.15 M. The obtained mildly oxidized graphene oxide can be used to produce a water-based suspension of good time stability (without any visible temporal changes for one month). The films prepared from the mildly oxidized graphene oxide suspension by the drop casting method have sheet resistance 102-103 kΩ/sq, depending on film thickness. The thermal reduction at a relatively low temperature of 300 °C allows obtaining conductive films with a sheet resistance of about 1.5 kΩ/sq for a thickness of about 100 nm. The thin films (∼16-20 nm) obtained by inkjet printing or the use of spin-processor have their sheet resistance of 10-60 kΩ/sq and carrier mobility of 5-7 cm2 V-1 s. The mildly oxidized graphene oxide suspension is promising for preparing stable water-based inks to be used in conductive printed layers and flexible electronics.