When I wrote the comment above, my memory was blanking. The connection between ring maps $MU\to R$ and complex orientations that Mark describes goes back to Quillen's original work relating $MU$ to formal group laws. (Lemma 4.6, page 52, in Adams Stable Homotopy and Generalized Cohomology characterizes ring maps $MU\to R$ in terms of complex orientations).
A discussion of $E_{\infty}$ orientations $MU\to R$ that may help with the original
question is given in Section 5 of ``What are $E_{\infty}$ ring spaces good for?'', which relates spectrum level orientations to $E_{\infty}$ orientations on the space level. Such orientations are equivalent to $E_{\infty}$ maps $BU \to B(U;R)$, where $B(U;R)$ classifies $R$-oriented $U$-bundles (in the classical sense of orientation). I apologize if the relevance is unclear. It's late.
[Continued] Here is some more background. To understand the mathematics here, you must recognize that the unit space once called $F$ and now called $GL_1(S)$ plays two very different roles, one additive and one multiplicative. This is explained in the introduction to "What are $E_{\infty}$ ring spaces good for?" The space $BF$ classifies sectioned stable spherical fibrations, and its product classifies fiberwise smash products. You go from stable vector bundles to stable spherical fibrations by fiberwise one point compactification, and that takes Whitney sum of bundles to fiberwise smash product. Therefore we think of this as additive structure. The map $BU \rightarrow BF$ sees this on the represented functor level,
and it is maps like this that you are thinking of as leading to Thom spectra (as they do by
Gaunce Lewis's thesis, in "Equivariant stable homotopy theory" LMS 1213. There is also an infinite loop space $BU_{\otimes}$, which is the identity component of the space $GL_1(KU)$. There is no analogous $BF_{\otimes}$.
As the unit space $GL_1(S)$, we think of the same space $F$ as multiplicative. For any
commutative (ie $E_{\infty}$) ring spectrum $R$, we have the unit map $S\to R$, and
on passage to zeroth spaces it induces an infinite loop map $GL_1(S) \rightarrow GL_1(R)$.
There is a fibration sequence $B(U,R) \to BU \to BGL_1(R)$. The first map says ``forget the orientation''. The second is the obstruction to (universal) $R$-orientability of complex vector bundles. An $E_{\infty}$ $R$-orientation of complex bundle theory is an $E_{\infty}$ map $g\colon BU \to B(U,R)$ which sections $B(U,R) \to BU$. Such orientations $g$ correspond to maps $MU\to R$ of commutative ring spectra, as I explained in Section 5 opus cit. Of course, everything in this general theory works equally well for other kinds of bundles. The interplay of additive and multiplicative structure is the key to the splitting of $SF$ as $J\times Coker J$ at each prime $p$ (only an $E_{\infty}$ splitting when $p$ is odd).
