It's not officially confirmed yet, but it is said that the scenario of the OAD was written by Suga Shotaro, who was in charge of series composition of the latter part of S1 and wrote scripts of 7 episodes in total (
ANN). Simply, he was a main writer of S1, though he was not involved with making S2 yet probably because of the OAD work. So I assume that the OAD would be more like S1 rather than S2.