The 24th of June, the Debian package for new upstream OCaml 3.11.1 has been uploaded. Thus upload marks the start of the transition to OCaml 3.11.1 in Debian Sid (aka unstable). Right now, the new package has been successfully built on most of Debian supported architectures.
Following this first round, other packages are being uploaded in successive rounds. You can follow this transition on Stéphane Glondu's dedicated OCaml transition monitor.
The count-down has started. We will see how much time it will take to do this transition for the 138 source packages.
 By the way, it appears Stéphane's code will be used in Ubuntu to monitor transition for Haskell packages.