The Oblique ECE diagnostic installed at JET allows simultaneous analysis of the ECE spectra along three lines of sight (with toroidal angles 0°, ~10°, ~20°) and 2 linear polarizations for each oblique line of sight. Capable of measuring EC emission over the band 75-800GHz with 5ms time resolution and 7.5GHz spectral resolution, the diagnostic is designed to investigate the features of ECE spectra related to electron distribution in the thermal velocity range. Instrumental accuracy was assessed using sources at different temperatures (77-900K) and with plasma emission. ECE from high temperature plasmas and in presence of fast ions has been compared to simulations performed with the modeling code SPECE, putting an upper limit to possible discrepancies from thermal emission.