or so vs approximately

or so and approximately both are adverbs.

Word	Noun	Adjective	Verb	Adverb
or so	No	No	No	Yes
approximately	No	No	No	Yes

As adverbs, or so and approximately are synonyms defined as:

or so and approximately: (of quantities) imprecise but fairly close to correct

Other synonyms of or so include about, around, close to, just about, more or less, roughly, some.

or so (adverb)	approximately (adverb)
(of quantities) imprecise but fairly close to correct	(of quantities) imprecise but fairly close to correct