Which of these compounds are oxoacids (also known as oxyacids)? Ba(OH)2
HCN
HF
nitrous acid
H2PO4
hydrochloric acid
chloric acid

At the very least, an oxoacid must 1) be an acid and 2) contain oxygen.

Ba(OH)2 (barium hydroxide) is a strong base. HCN, HF, and hydrochloric acid (HCl) don't contain oxygen.

Nitrous acid (HNO2) is an acid, and it contains oxygen. And the acidic hydrogen is bonded to an oxygen. Thus, nitrous acid qualifies as an oxoacid.

I don't know if H2PO4 is a typo; if not, then it would properly be written as H2PO4⁻ since it's the first deprotonation product of phosphoric acid, H3PO4. In any case, H2PO4⁻ is still acidic, albeit weakly, and its acidic hydrogens are bonded to the oxygen atoms. Thus, "H2PO4" would qualify as an oxoacid (for that matter, H3PO4 would also be an oxoacid).

Chloric acid (HClO3) is an acid, and it contains oxygen; its acidic hydrogen is bonded to an oxygen atom. Thus, chloric acid qualifies as an oxoacid.