iwasawa theory - When does a p-adic function have a Mahler expansion?

Let $f: mathbb{Z}_p rightarrow mathbb{C}_p$ be any continuous function. Then Mahler showed there are coefficients $a_n in mathbb{C}_p$ with

$$
f(x) = sum^{infty}_{n=0} a_n {x choose n}.
$$

This is known as the Mahler expansion of $f$. Here, to make sense of $x choose n$ for $x notin mathbb{Z}$ define

$$
{xchoose n} = frac{x(x-1)ldots(x-n+1)}{n!}.
$$

There's an important application to number theory: expressing a function in terms of its Mahler expansion is one step in translating the old fashioned interpolation-based language of $p$-adic $L$-functions into the more modern measure-theoretic language. This application is explained in Coates and Sujatha's book Cyclotomic Fields and Zeta Values.

However, when the $p$-adic $L$-function is more complicated than Kubota-Leopoldt's, it seems to me that this "translation" really requires one to be able to write down a Mahler expansion of a function $f$ with much larger domain, e.g. the ring of integers of the completion of the maximal unramified extension of $mathbb{Q}_p$ or some finitely ramified extension. (See, for instance, line (8), p. 19 of de Shalit's Iwasawa Theory of Elliptic Curves with Complex Multiplication).

I can't find a published reference that these functions really have Mahler expansions. Mahler's paper uses in an essential way that the positive integers are dense in $mathbb{Z}_p$, so it doesn't instantly generalize.

So is it true or false that for a ring of integers $mathcal{O}$ in a finitely ramified complete extension of $mathbb{Q}_p$, and a function $f: mathcal{O} rightarrow mathbb{C}_p$, there is a Mahler expansion as above?