  Zhao and the second author (2013) constructed a functor from o(k)-Mod to o(k
+ 2)-Mod. In this paper, we use the functor successively to obtain an universal
first-order differential operator realization for any highest-weight
representation of o(2n + 3) in (n + 1)^2 variables and that of o(2n + 2) in n(n
+ 1) variables. When the highest weight is dominant integral, we determine the
corresponding finite-dimensional irreducible module explicitly. One can use the
result to study tensor decompositions of finite-dimensional irreducible modules
by solving certain first-order linear partial differential equations, and
thereby obtain the corresponding physically interested Clebsch-Gordan
coefficients and exact solutions of Knizhnik-Zamolodchikov equation in WZW
model of conformal field theory. We also find an equation of counting the
dimension of an irreducible o(k + 2)-module in terms of certain alternating sum
of the dimensions of irreducible o(k)-modules. In the case of the Steinberg
modules, we obtain new combinatorial identities of classical type.
