$ opam list --roots
will give you packages that you explicitly installed.
List only packages that were explicitly installed, excluding the ones installed as dependencies
$ opam list --depends-on=myPackage
should give you packages that depend on myPackage
List only packages that depend on one of (comma-separated) PACKAGES.
$ opam list --required-by=myPackage
should give you the dependencies of myPackage
List only the dependencies of (comma-separated) PACKAGES.