Oblique Attacker (CO, p.78) states that the unit can fire at a target indirectly without a spotter, and get a -1 to hit when firing indirectly.
'Mech Mortars (TO:AUE p.136) state that they can be fired at a target indirectly without needing a spotter, but at a +2 penalty if no spotter is present.
Ask: If a unit with 'Mech Mortars also has the Oblique Attacker SPA, do the mortars still suffer the +2 "no spotter present" penalty when firing indirectly?