We will report the results of the target of opportunity (ToO) observations carried out hours after the GRB 100316A with the OSIRIS instrument. These observations represent the first spectroscopic ToO executed at GTC for a GRB afterglow. The observations, performed with the low resolution R300B and R300R grisms of OSIRIS, revealed a smooth continuum truncated by a sharp Lyman-alpha break. A careful analysis of the trace reveals a faint Lyman-alpha emission slightly off from the afterglow trace. We interpret this emission as the Lyman-alpha emission coming from the host galaxy at redshift z=3.1.