To create an interesting plot in supernatural OC God fanfiction, consider the conflict. The God could be in conflict with other supernatural beings, and the OC gets caught in the middle. Let's say there are two rival Gods, and the OC has to choose a side. This creates a moral dilemma for the OC. You can also add some mystery, like the God's true identity is hidden, and the OC has to figure it out during the story.
Well, start with the world - building. In a supernatural fanfiction with a God, the world has to be different from ours. Decide on the rules of this world. For the OC, give them a distinct personality and backstory. If the God in your story has the power of prophecy, the OC could be someone trying to avoid a foretold doom. Build the plot around how they interact with the God's powers and intentions.