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We present a spectral study of the single magnetically active K giant OP And in the period 1979 -- 2018, monitoring the variability of the activity indicator line H$α$. Original data obtained in the period 2015 -- 2018 with the echelle spectrograph ESpeRo at the 2m telescope of the National Astronomical Observatory Rozhen in Bulgaria, previously unpublished original data obtained in the period 1997 -- 2007 and on one night in 2013 with the Coude spectrograph at the same telescope, as well as data from the literature are presented in this study. The variability of the H$α$ line reveals that the activity level of OP And is higher in the period 1993 -- 2000, while during the period 2008 -- 2010 it is lower, possibly close to a minimum. Also, our data for the period 2015 -- 2018 show that the activity level is increasing again. Spectral observations of the activity indicators CaII H&K lines and CaII IR triplet are sparse during the studied period. We use such ones when possible to confirm the detection of some flare events. The structure of H$α$ changes with the activity level: when the activity is higher, we observe a blue-shifted component of this line, interpreted as an expanding area above the photosphere, but during a lower activity period it is almost absent. Our results are in a good agreement with the idea that the magnetic field controls the mass outflow in this giant. More years of observations are necessary to determine the eventual activity cycle of OP And.