The OCaml manual says “No attempt has been made at mathematical rigor: words are employed with their intuitive meaning, without further definition. As a consequence, the typing rules have been left out, by lack of the mathematical framework required to express them, while they are definitely part of a full formal definition of the language.”.
I suspect that OCaml types can be described in plain English and I propose to attempt at least in part.
I think also that plain English can be rigorous.
That is not easy however.

I skip records (6.2.3) because there are details that I have not guessed.
Concerning “Polymorphic Variants” (6.2.6), show me one.
I think that the eight categories of values (I hesitate to use the term ‘type’ here) are disjoint.
Regarding tuples:
For each integer n > 1, for each sequence of n types where α<sub>j</sub> is a type-expression denoting the jth of these types, there is a type-expression ⸢(α<sub>1</sub> * ... * α<sub>n</sub>)⸣ which denotes  the type of tuples whose jth element is of type α<sub>j</sub>.
<p>For each n≥0 and for each type denoted by type-expression α there is a type denoted by type-expression