Based on the long-term data in the literature, we present evidence for long-term optical spectral index variability behaviour for BL Lacertae object OJ 287. First we find that the optical spectral index variability period of BL Lacertae object OJ 287 is in agreement with the optical flux variability period of about 11.96yr. It is considered that different processes induce optical variability, and each process should have a definite signature in the spectral index, thus leading to the periodicity of the spectral index. We also find that the time lag between the optical spectral index and the optical flux density is half of the optical flux variability period. This result tends to support the argument that when the brightness of the long-term variations increases, the optical spectra become bluer. These variations can be explained by energy loss of the electrons during the outburst. We show that the spectral index is different during the flares compared with outside the flares; between the first and second flare peaks of the superflares, the spectrum shapes are different. Disparate processes possibly induce the two peaks of OJ 287.