so, I'm really liking everything I've seen in the warlock class so far and I'm trying to make myself one with the Great Old One as my patron but I'm struggling with the specific patron. A little specifics about my character he's going to be a ranged blaster with a good deal of diplo/bluff. I really like the GOO option for telepethy and a thrall at 14, so I'm not much interested in the other two options right now. I also want him to be relatively well aligned with the patrons goals so I can try to convince party members/NPCs to also make pacts with him. The main part I'm having trouble with is that most of the GOO patrons in the example want to end the world/all existence and I'm not so much down for that. I was thinking mayBe AO from forgotten realms would work, but I'm having trouble coming up with a decent reason for him to even know about my characters existence much less be willing to forge a pact with me. So here I am asking for help, what patron are you guys choosing(if you are looking at GOO) or are there any other creatures from a different fantasy realm (or from D&D that maybe I'm not familiar with) that would qualify as a GOO who isn't bent on the destruction of the world?
I say let the Great Old One be bent on the destruction of the world... only it's fooled your character into believing it is something benevolent. So your warlock happily does its bidding, never realizing what you're doing serves this unspeakable being's fell purpose. At some point (when it's time to retire the character), you realize the truth and it drives you insane.
I'd go with Hastur, The King in Yellow.
The PHB say the great Old One might not even know your warlock exists, but you know secrets of drawing on their power. The only thing you need to figure out is how your warlock found these secrets, and how they can convince others to tap into this power supply.
Well, for a character motivation for worshipping an Old One, I'd suggest something loosely akin to many fictional versions of the Thuggee - they are often portrayed as a cult of Kali worshipers, who are occasionally said to believe that killing in her name delays her coming into the world, and thus the destruction of it. Thus, by serving/appeasing this being, he or she truly believes that they're saving the world from a much worse fate.
Or, alternately, they are fully dedicated to the destruction of the world as we know it - However, they justify the destruction of the current reality by the belief that they're ushering in a much better world. (With no right angles and non-euclidian frogs, lol)
Given the eons-long existence and alien thought processes of Old Ones, the character could theoretically live an entirely Lawful Good life and still be a major pawn in the destruction of the known universe, lol.
The Warlock doesn't necessarily *worship* their pact patron. And the Great Old One pact can be made via finding old tomes of secret and forgotten knowledge which tell you how to manifest the power of the pact. You never even have to summon the entity directly, just suck up their GOO juices.
(Yes, I came into this thread specifically to use the term GOO juices, since you titled the thread as 'GOO Warlock flavor help'... 
)
Lots of great advice. Thank you everybody. It seems that in my haste I missed the line that states your patron might not be aware of you, so I think I'm going to go with AO if my DM allows and just say that I stumbled across a ritual that allowed me to tap into his power a little bit, that way I can still try to convert everybody I come across should I choose. Also, I like the idea of being able to say to a cleric "my god is stronger than yours"
Id steal Tzeenech, Keeper of Secrets from warhammer 40k lore, and tweak him a bit, for great old one pact.
If you can get Dragon Magazine 381, there's good article on this very subject, as well as an example star pact patron by the name of Ulban. He's actually a shooting star that's seen the end of the universe, and forms pacts with warlocks in an attempt to avert it. Its worth checking it out.