Also, if you use a mod (like Factorio World) that replaces water tiles, you can manually add it to this mods info.json as a dependency, that way the oil will be placed after the other mod places water tiles. (@schnurrebutz Might be good to add some of those mods as optional dependencies, not sure what others)
Example info.json change:
"dependencies": ["base >= 0.16", "? factorio-world"],
Logically, this would be a responsibility of mods like factorio-world to load/run first, but I don't think the mod api allows for that.