The general form of the Van der Pol ODE is given by the following:

Van_der_Pol_ode := diff(y(x),x,x)-mu*(1-y(x)^2)*diff(y(x),x)+y(x)=0;

See Birkhoff and Rota, "Ordinary Differential Equations", p. 134.

The second order Van der Pol ODE can be reduced to a first order ODE of Abel type as soon as the system succeeds in finding one polynomial symmetry for it (see ?symgen):

with(DEtools, odeadvisor, symgen):

odeadvisor(Van_der_Pol_ode);

symgen(Van_der_Pol_ode, way=3);

From which, giving the same indication directly to dsolve you obtain the reduction of order

ans := dsolve(Van_der_Pol_ode,way=3);

For the structure of the solution above see ?ODESolStruc. Reductions of order can also be tested with odetest

odetest(ans,Van_der_Pol_ode);

The reduced ODE is of type Abel, and can be selected using either the mouse, or the following:

reduced_ode := op([2,2,1,1],ans);

odeadvisor(reduced_ode);