Another idea that will get you part of the way there is to use refmt to automatically format the OCaml files to standard layout.
This would cover 3 out of the 4 criteria because:
1. it automatically indents
2. it always reduces parentheses where it can
3. it automatically converts begin and end to parentheses.
The 4th criteria could potentially be caught using grep.
It's worth an experiment to see if you like the style of code generated by refmt for OCaml. It certainly does a better job at formatting reason code from my limited experiments (it doesn't seem to like vertical white space when generating OCaml, whereas it will include it if you covert the file to reason, testing version 1.13.5 as of writing).