Is there a rule for when to use "w" and when to use "ou" for you? I notice that "w" is used sometimes even when the sound is still "ou," like: "M renmen w." Are they completely interchangeable?

w is the contracted form of ou (as a pronoun and possessive adj)
For many foreigners the difference between the two (when you listen to them) is very subtle.  But if you listen to the H. Creole language long enough you'll soon be able to distinguish between the two sounds.

Yes there are rules.
When used as an object pronoun or possessive adjective, it follows the vowels not the consonants.
example:
you can say:
Mwen renmen w. ("w" follows nasal vowel "en" in "renmen")
Nou sonje w. ("w" follows vwayèl-bouch "e" in "sonje")
Papa w pa pè. ("w" follows vwayèl-bouch "a" in "papa")

But you cannot say:
M'ap tann w. ("w" cannot follow the consonant "n" in "tann")
Li bat w. ("w" cannot follow the consonant "t" in "bat")
Nou fè lwanj w chaj jou. ("w" cannot follow the consonant "j" in "lwanj")
Eske sa se kay w? ("w" cannot follow the consonant "y" in kay)

We'll say instead:
M'ap tann ou.
Li bat ou.
Nou fè lwanj ou chak jou.
Eske sa se kay ou?

When in doubt, you can always use "ou".

FYI:  In Northern Haiti, however, this rule goes out the window.  The "w" will sounds like "a ou" or "aww"
The Northerner will say:
Nou fè lwanj w chak jou.
And it will sound like:
Nou fè lwanj aww chak jou.

Check out these posts:  OU and W

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words