How do I check that, where in settings would it be?
Connect a pair of headphones and see if it plays on both sides.
Also, try removing and pairing the ONN earbuds. Remove the ONN Bluetooth from the pairing list on your device. In some devices, you must select to "forget the headphones" to delete them from your device's paired lists.
Pair the ONN Bluetooth earbuds to reestablish the connection.