If I cast Oppressive Ray on an opponent creature, Am I the one controlling the enchantment even if it's on his creature? (Will it count as one on my enchantment if have an Helm of the God?)
Yes, you still control the Oppressive Rays.

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If I cast Oppressive Ray on an opponent creature, Am I the one controlling the enchantment even if it's on his creature? (Will it count as one on my enchantment if have an Helm of the God?)
Yes, you still control the Oppressive Rays.
If I cast Chandra's Ignition on Zurgo Helmsmasher and it kills 3 of my creatures and 4 of the opponent creatures. Is it correct that Zurgo will gain 7 +1/+1 counters?
Yes.