Let Q be a nondegenerate quadratic form over the nite eld Fq of odd prime power order q with charFq 6= 2 and Let On(Fq) be the associated orthogonal group. Let On(Fq) act linearly on the polynomial ring Fq[x1; : : : ; xn]: In this thesis we nd the invariant subring Fq[x1; x2; x3; x4; x5]O5(Fq ) with explicit generators. We also prove that this subring is a UFD.