When to use œ or ø??

So, now I know that they sound different. But I've failed to find grammar rules about when to use either one of them.

For instance I don't understand why eu in "euro" and "heure" sound diffrent.

Don't they both sould have closed eu sound?

I read somewhere it depends on ending like in the case of "œuf" and "œufs" But both euro and heure end in vowel so I' lost here

Thank you for reading my question :)

And one more thing. Is "oe" always technically should be written "œ"? I'm confused if only in certain words it becomes œ or everytime when o and e come together it should be œ