ものとする

ものとする is the "shall" of contracts, rules and regulations: it lays down what is to be done. It goes after a verb in the dictionary form or the ない-form, and it almost never leaves formal writing.

It is milder than なければならない: it states the rule as the agreed procedure rather than ordering someone to follow it.

ものとする or こととする

こととする announces a decision: "it has been decided that". ものとする states a standing rule. In practice both appear in official notices, and こととする is the lighter of the two.

Usage notes

These go beyond laying down a rule.

Deemed: 〜たものとする

After a た-form, ものとする says how something will be treated, whether or not it literally happened: "shall be deemed".

ものとみなす says the same thing more explicitly.

Assume that

In a maths or science problem, ものとする sets a condition for the calculation: "assume that".