The orientations are actually QRS, -PRS, PQS, and -PQR. The reason is that the edges need to "cancel" so to speak. Imagine an orietation as a path to be walked along. So QRS means "starting at Q, walk to R, then walk to S". Each edge is contained in exactly two faces, so it is transversed twice, and it must be transversed in opposite directions. So the edge PQ is contained in the PQR face and the PQS face, but it is transversed in the same direction both times (P to Q).

By the way, the oriented faces of a simplex with vertices are where . (do you know what the boundary homomorphism is btw?)