Opponent has Ornn with deflect 2 at a battlefield and a recruit in base. I want to use Piercing Light, targeting the Ornn and the recruit. Since Ornn has deflect, do I have the option to not use the Piercing Light on the Ornn and only on the recruit?

Yes, you have the option to not target the Ornn, Forge God.

Piercing Light says: "Deal 2 to a unit at a battlefield, then deal 2 to up to one other unit."

Because the second part of the spell specifies "up to one other unit," you are not required to select a second target. When you play Piercing Light, you can choose to target only the recruit in the base if you wish.

However, there is a technical constraint regarding your specific goal: Piercing Light requires the first target to be a unit at a battlefield. If you choose to play the spell, you must target a unit at a battlefield as the first requirement. If the only unit at a battlefield is the Ornn, and you do not wish to pay the Deflect 2 cost, you cannot play the spell targeting that Ornn.

To summarize:

1. If you have another unit at a battlefield available to target, you can target that unit first, and then choose not to target the Ornn at all for the second part of the spell (or target the recruit).
2. If the Ornn is the only unit at a battlefield, you cannot legally play Piercing Light without choosing it as the first target, which would trigger its Deflect 2 cost.