  In this paper, we study Einstein gravity with a minimally coupled scalar
field accompanied with a potential, assuming an O(4) symmetric metric ansatz.
We call an Euclidean instanton is to be an oscillating instanton, if there
exists a point where the derivative of the scale factor and the scalar field
vanish at the same time. Then we can prove that the oscillating instanton can
be analytically continued, both as inhomogeneous and homogeneous tunneling
channels. Here, we especially focus on the possibility of a homogeneous
tunneling channel. For the existence of such an instanton, we have to assume
three things: (1) there should be a local maximum and the curvature of the
maximum should be sufficiently large, (2) there should be a local minimum and
(3) the other side of the potential should have a sufficiently deeper vacuum.
Then, we can show that there exists a number of oscillating instanton solutions
and their probabilities are higher compared to the Hawking-Moss instantons. We
also check the possibility when the oscillating instantons are comparable with
the Coleman-DeLuccia channels. Thus, for a general vacuum decay problem, we
should not ignore the oscillating instanton channels.
