This site claims that Osem (אֹסֶם) is a term taken from an ancient Jewish prayer. What prayer would that be?
Anectodote: I had assumed it was a Hebrew transcription of the English word awesome due to the company's slogan It's Good. It's Osem.
While the more familiar 'Yehi Ratzon' supplication given by this answer is probably the source of the company's name it may be worth noting another usage from antiquity of this precise form.
In a poem to be sung on the eve of RH & YK, R. Ephraim of Bonn, 12th c. wrote ('Piyyute R. Ephraim br' Yaakov M'Buna' p.44):
שנת אסם וברכה באו אלי כעת מחר לחננה
(Unfortunately, I cannot locate a link online.)
Parenthetically, Ilan Adler (Leshonenu, 1976 p.76) suggests that, contrary to the vocalization by most dictionaries for the singular as 'uh-sum', the proper form should be 'ah-sum' (either way, it's not 'oh-sem').