Executing rustc -C help shows (among other things):
-C opt-level=val -- optimize with possible levels 0-3, s, or z
The levels 0 to 3 are fairly intuitive, I think: the higher the level, the more aggressive optimizations will be performed. However, I have no clue what the s and z options are doing and I couldn't find Rust-related information about them.
These two sequences, Os and Oz, within LLVM, are pretty similar. Oz invokes 260 passes (I am using LLVM 12.0), whereas Os invokes 264. Oz' sequence of analyses and optimizations is almost a strict subsequence of Os', except for one pass (opt -loops), which appears in a different place within Os. This said, notice that the effects of the optimizations can still be different, because they use different cost models, e.g., constants that determine the behavior of optimizations. Thus, optimizations that have impact on size, like loop unrolling and inlining can behave differently in these two sequences.