For a stationary state the probability density does not oscillate in time.
The wavefunction of a stationary state does evolve in time according to ψ(x,t)=f(x)e-iEt, where f(x) is an eigenfunction of the Hamiltonian, and E is the corresponding eigenvalue. However, the probability density is the "square" of the wavefunction, ψψ*, where the multiplication of e-iEt with the complex conjugatate of eiEt gives e0=1, which doesn't change with time.
The probability density of a general state does evolve in time, because it is the superposition of several eigenfunctions.