A detailed zero-and transverse-field muon spin rotation investigation of magnetism and the magnetoelectric coupling in Cu2OSeO3 is reported. An internal magnetic field B-int(T = 0) = 85.37(25) mT was found, in agreement with a ferrimagnetic state below T-c = 57.0(1) K. The temperature dependence of the magnetic order parameter is well described by the relation B-int = B(0)(1 - (T/T-c)(2))((beta) over tilde) with an effective exponent (beta) over tilde similar or equal to 0.39(1), which is close to the critical exponent beta similar or equal to 1/3 for a three-dimensional (3D) magnetic system. Just above T-c the muon relaxation rate follows the power law lambda(T) proportional to (T/T-c - 1)(-(omega) over tilde) with (omega) over tilde = 1.06(9), which is characteristic for 3D ferromagnets. Measurements of B-int(T) with and without an applied electrostatic field E = 1.66 x 10(5) V/m suggest a possible electric-field effect of magnitude Delta B-V = B-V(0 V) - B-V(500 V) = -0.4(4) mT.