Big O's on bases less than 1

Question

I have a function $f(x) = x^3 + .92^x$. I would assume that it is $\mathcal{O}(n^3)$ because $.92^x$ exponentially decays as $x$ grows and $x^3$ continually grows, but it is $\mathcal{O}(.92^x)$. I'm not entirely sure why. I would get it if the base was greater than $1$, because obviously an exponential grows faster than a cubic, but it isn't.