We present results from five nights of pseudo-simulations JHK infrared monitoring of the blazar OJ 287 taken with the Carlos Sanchez Telescope during March 1991 with a time resolution of approximately 20 min. The monitoring shows small amplitude variations in time scales of a few tens of minutes, which are largest in J. Considerable variations are seen in the spectra and the spectral index in time scales comparable with our time resolution. This is the first time that spectral variations have been observed at any wavelength on such short time scales on OJ 287. There is a strong correlation between the spectral index and the intensity in the J-band which also shows significant night to night variations. We can rule out the cause of the variations being instrumental, or due to sky transparency variations, with a high degree of confidence. The spectrum is seen to tilt about a point at approximately 1.9 micrometers, although the exact value of the wavelength of the pivotal point varies from night to night. These observations could be explained by a large, variable magnetic field, in which the energy spectrum of injected relativistic electrons varies due to radiative decay on time scales of a few minutes. An alternative, but less viable explanation, is a two-component model in which a non-variable component dominates the flux in K and H bands, but the flux is dominated by a variable emission in the J-band.