There is nothing unusual in OCP  being negative.  It can be of either sign.  It all depends on the reference electrode and in your case it is negative wrt to Ag/AgCl.  This is due to fact that there is no way of measuring absolute potential difference of any electrode and you always have to measure wrt some reference electrode  ( See for example Bockris and Reddy, Modern Electrochemistry Vol 2). It can turn more negative under illumination  especially for some semi conductor electrodes due to variation in charge carrier density at the interface.

There is nothing unusual in OCP  being negative.  It can be of either sign.  It all depends on the reference electrode and in your case it is negative wrt to Ag/AgCl.  This is due to fact that there is no way of measuring absolute potential difference of any electrode and you always have to measure wrt some reference electrode  ( See for example Bockris and Reddy, Modern Electrochemistry Vol 2). It can turn more negative under illumination  especially for some semi conductor electrodes due to variation in charge carrier density at the interface.

The OCP vs AgCl will first of all depend on the electrolyte (redox couple, pH etc...) you are using. Variations in time of the OCP are further determined by the impedance of the measuring equipment, although this is very large (in general >1012 ohm) there is always some current flowing so there is no such thing as an absolute OCP, the fact of measuring influences the potential. If you are using n-type semi conductor electrodes electrons are excited in the conduction band upon illumination resulting in a more negative potential. Or if you are using a photosensitive redox couple a change in concentration of oxidized and reduced species upon illumination can produce similar effects.

Thank you all for your kind answers.