Spiritual weapon does not get disadvantage from the oathbow's effect, but other summoned weapons may.
The "Weapons" portion of the basic rules states:
Your class grants proficiency in certain weapons, reflecting both the class’s focus and the tools you are most likely to use.
Weapons are tools that characters can gain proficiency in.
The description of the spiritual weapon spell states:
You create a floating, spectral weapon within range that lasts for the duration or until you cast this spell again. When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon.
A spiritual weapon attack is a melee spell attack (which the oathbow does not apply to), not a ranged weapon attack or a melee weapon attack (which the spell would need to give you proficiency for).
Further adding to this point is the shadow blade spell (XGtE, p. 154), which says:
It counts as a simple melee weapon with which you are proficient.
Shadow blade specifically calls out that it is a simple melee weapon with which you are proficient.
Magical summoned weapons of this type would make attacks at disadvantage due to the oathbow's effects.