Multi-User Multiple-Input Multiple-Output (MU-MIMO) cellular systems with an excess of Base Station (BS) antennas (Massive MIMO) offer unprecedented multiplexing gains and radiated energy efficiency. Oscillator phase noise is introduced in the transmitter and receiver radio frequency chains and severely degrades the performance of communication systems. The authors study the effect of oscillator phase noise in frequency-selective Massive MIMO systems with imperfect Channel State Information (CSI). In particular, they consider two distinct operation modes, namely when the phase noise processes at the BS antennas are identical (synchronous operation) and when they are independent (non-synchronous operation). They analyze a linear and low-complexity Time-Reversal Maximum-Ratio Combining (TR-MRC) reception strategy.