Question

Why sulphur exist as a S8 but oxygen exit as a O2

Answer 1

because of the bond formation and valancy of the elements

Answer 2

oxygen exist as O2 bcoz it has an electronic configuration as 2,6.
it requires 2 electrons to fill its octet.this achieves by sharing of an another oxygen atom. hence a double bond between two oxygen atoms forming an oxygen molecule represent the sharing of 4 electrons.
whereas sulphur has an electronic configuration as 2,8,6.
to attain the electronic configuration of the nearest noble gas each sulphur needs 2 electrons.