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In this letter, we describe the impact of quadrature imbalance (QI) in the presence of frequency offset and phase drift in an optical coherent offset-quadrature phase-shift-keying (OQPSK) receiver.In the time domain, QI tilts and squeezes the square constellation into a diamond shape.What more, frequency offset continuously rotates the constellation.In the presence of both QI and frequency offset, the constellation becomes elliptical.Arbitrary conjugate misalignment was realized in a 2×4 90° optical hybrid, and the Gram-Schmidt orthogonalization procedure (GSOP) was applied in our simulation to compensate for quadrature imbalance in the hybrid.Firstly, the GSOP enabled a set of nonorthogonal samples to be transformed into a set of orthogonal samples.Secondly, a time domain method was adopted to compensate for chromatic dispersion (CD) due to fiber transmission.Thirdly, the frequency offset between the received optical signal and LO signal, which was 200MHz in our simulation, was estimated and corrected by the phase increment estimation algorithm.Finally, we recovered the ideal square constellation after carrier phase estimation.