Why radiator mix oil and water

@watban most common cause is a damaged cylinder head gasket. This allow oil and water to mix and can also be the cause for the overheating. I suggest that you stop driving your Honda since you may end up with further damage.

Replacing a cylinder head gasket (or even a cracked cylinder head) is pretty involved and if you have never done this, you could consider some help from a mechanic, shop etc.

For a more specific breakdown and how to replace it, we would need to know the exact year and engine size for your Honda.