Conflicting installation advice

On https://ocaml.org/docs/install.html it says:

OPAM
 
OPAM is a package manager for OCaml. It is the recommended method to 
install the OCaml compiler and OCaml packages.

Nice.  One specialised installer to install everything ocaml, which 
enables me to keep things more up-to-date than my distro might.
 
And on https://opam.ocaml.org/doc/Install.html it says the preferred way 
to install opam is from my distro's package repository.

So thats the bootstrap mechanism.

But when I ask aptitude to install opam, it also installs a huge number 
of other packages, such as ocaml.  Not surprising, because opam installs 
packages from source.

This would seem to put ocaml, and all those other packages outside the 
scope of opam.  Too late to install ocaml using opam.

If I persist in using opam to install ocamland other libraries, how do 
I distinguish the two installations -- the one by opam and the one  
from aptitude?

-- hendrik