Step 1: Assume the required circle.
Since the circle passes through the origin, its equation may be written as
\[
x^2+y^2+2gx+2fy=0.
\]
Step 2: Use orthogonality with the first circle.
For
\[
x^2+y^2+6x-15=0,
\]
we have
\[
g_1=3,\quad f_1=0,\quad c_1=-15.
\]
The condition of orthogonality is
\[
2gg_1+2ff_1=c+c_1.
\]
Thus,
\[
6g=-15.
\]
Hence,
\[
g=-\frac52.
\]
Step 3: Use orthogonality with the second circle.
For
\[
x^2+y^2-8y-10=0,
\]
we have
\[
g_2=0,\quad f_2=-4,\quad c_2=-10.
\]
Applying the orthogonality condition,
\[
-8f=-10.
\]
Therefore,
\[
f=\frac54.
\]
Step 4: Form the required circle.
Substituting the values of \(g\) and \(f\),
\[
x^2+y^2-5x+\frac52y=0.
\]
Multiplying throughout by \(2\),
\[
2(x^2+y^2)-10x+5y=0.
\]
Step 5: Final conclusion.
Hence the required circle is
\[
\boxed{2(x^2+y^2)-10x+5y=0}.
\]