The monthly time series of carbonyl sulfide (OCS) atmospheric mole fractions measured at NOAA network stations (2000 to 2020) have been analyzed, and the long-term behaviour has been assessed based on the Empirical Mode Decomposition (EMD). EMD is a fully non-parametric analysis of frequency modes and trends in a given series and is based on the data alone. We have found that the OCS atmospheric mole fraction, after an increasing phase up to ~2015, with a temporary decline around 2009, is now decreasing at all stations, reflecting a recent imbalance in its total sources and losses. Our analysis has revealed a characteristic time scale for variation of 8-10 years. The variance associated with this long-term behaviour ranges from 15 to 40% of the total strength of the signal, depending on location. To our knowledge, this low-frequency mode is a novel result not assessed in previous studies. Apart from this complex long-term behaviour, the OCS time series show a strong annual cycle, which primarily results from summertime OCS uptake by vegetation. In addition, we have also found one more frequency of minor variance intensity in the measured mole fraction time-history, which corresponds to periods in the range of 2 to 3 years. This inter-annual variability of OCS may be linked to the Quasi-Biennial Oscillation or QBO.