What is the oxidation no. of oxygen in AsO43-?

As a standard, think of oxygen at '-2'

NB There are one or two subtle variations, but they are not common.

For AsO4^(3-)

The anionic charge is '-3'

There are four oxygens at '-2' each , giving the oxygen moiety a total of '-8'

Summing

As + -8 = -3

Hence

As is + 5 ( oxid'n no.)