  It is known that scalar-tensor theory of gravity admits regular crossing of
the phantom divide line $w_{\DE}=-1$ for dark energy, and existing viable
models of present dark energy for its particular case -- $f(R)$ gravity --
possess one such crossing in the recent past, after the end of the matter
dominated stage. It was recently noted that during the future evolution of
these models the dark energy equation of state $w_{\DE}$ may oscillate with an
arbitrary number of phantom divide crossings. In this paper we prove that the
number of crossings can be infinite, present an analytical condition for the
existence of this effect and investigate it numerically. With the increase of
the present mass of the scalaron (a scalar particle appearing in $f(R)$
gravity) beyond the boundary of the appearance of such oscillations, their
amplitude is shown to decrease very fast. As a result, the effect quickly
becomes small and its beginning is shifted to the remote future.
