おおよそ + [number + counter] おおよそ + [verb] おおよその + [noun]
おおよそ is an adverb meaning "approximately", "roughly". It goes in front of a number, or in front of a verb to say something is roughly so. As a noun it takes の: おおよその is "rough", "approximate".
All three can go before a number. だいたい is the everyday word; おおよそ is a step more formal and turns up in reports, news and presentations. やく約 is the shortest and most written, and it only goes straight before a number – it cannot modify a verb.
Before a negative or a strong judgment, おおよそ means "utterly", "not remotely". This is literary, and the shorter およそ is the more usual form in this sense.