Can Oath of Fealty be used to kneel an opponent's knight?

Oath of Fealty

House Lannister Only

Marshalling: Kneel a Knight character to add 1 gold to your gold pool for each Knight character you control.

No. The first phrase "Kneel a knight character to" is a cost, and you can only pay costs with cards you control.

Thanks for clarifying, I appreciate it

This is probably obvious, but anything before a "to" is considered a cost in this game. I don't believe there are any instances in which this is not the case. So more generally, the part of an effect before "to" (the cost) can only be paid by game objects under your control (gold, cards, power primarily).

There may be an argument that you don't control power, but that's wayyyyyyyyyyyyyyy beyond the scope of this question.