OGL Horror is probably more like d20 Modern than d20 Call of Cthulhu (since it is built from WotC's Modern SRD).
There are four character classes in OGL Horror - Combatant, Scholar, Investigator, and Ordinary People. Each is a class with 10 levels (OGL Horror is a 10-level, not a 20-level game, and the characters start at level 3 by default). Each character also can choose a wide variety of occupations (similar to those found in d20 Modern) that provide additional benefits.
OGL Horror also has it's own, completely new rules for Horror Saves (three types - Panic, Fear, and Madness), Rituals (group spellcasting), Spellcasting, Psychic Abilities, and Faith effects.
It's a really great book, either on its own or as a d20 Modern toolkit. You are in for a real treat.