Is there any official OST of Ushio to Tora (2015)? I am pretty much interested in the soundtrack during battle scenes when Ushio calls the Beast Spear.
Assuming that the piece you're looking for is the one that is recreated in the Youtube video that Tyhja's answer links to, that's track #35 on the first volume of the soundtrack, titled "Yari no Otoko 2" ("Man with the Spear 2").
Note that the start of the recreated version of the piece corresponds to about 25 seconds into the actual track. Here is a 15-second extract: https://clyp.it/bdj1z10k.