This hasn't happened yet, but I'm tinkering around with the idea of introducing the insanity that a GOO is by having their very name be something that hurts the party. I'm going to describe it along the lines of "The occult leaders looks at you as you stab him through the side, peace overtaking him. As he falls over he gasps out a name, and when you hear it, terror fills your heart and pain floods your mind. Take x amount of psychic damage." If they ask me what the name was, I'll say the name itself is to slippery to hold onto. What do you think? And what would you do to make the GOO something the party fears without actually introducing an end of the world scenario?

I am currently Riyadh Cthulhu inspired campaign. Whenever a PC even starts trying to conceptualize WTF is going on I call for a Sanity Check and if they are below the DC they get a Short Term Madness. Actually having to interact with such the DC is higher and they are occasionally Saving Throws instead. Rack up enough short terms they get a Long Term Madness, currently 1/2 the party has one. A long term madness also grants them the Wild Talent feat that was in UA a while back before it died.)

I have an Aarakocra, Way of Shadow Monk, with Monster hunter background from a Lo5Rs world he is fleeing the "taint" madness.  and carries jade as an amulet.  checking to see if blackened.
As a  Aarakocra Stormcloud ''Lucas'' De'Valker, rolls a wisdom save.. if he has to go underground.. but does it with [advantge] if he has kicast darkvision.

"As the occultist fades, you see a sense of strangely blank peace settle over him.  His mouth moves, sound emerges, but the noises don't make sense.  They hit your ears with almost physical force, as if trying to invade your head.  The words are impossible to understand.  You almost feel like you're hearing shapes, textures - things that ears were never meant to perceive.  You understand that this is a name, some sort of moniker, but the occultist's words carry information your ears aren't built to process.  Roll an Intelligence saving throw."

Don't give them the actual name, or if you do, ensure that the words players know, remember, and speak are not the true name.  Merely a flat, pallid shadow missing much of the meaning of the true name.  Short-term madness is a good option, but I'd write up a custom madness table rather than using the generic shit in the DMG.  Suit it to your campaign, and to the Old One you're invoking.  And at all times, remember that the more you describe this critter and its related phenomena, the less Old it gets.  Never let your descriptions be clear or unambiguous.

Allips. Use allips. Somehow, someway, use them. Maybe a character slowly goes insane with a curse that will turn them into an allip within a certain time frame if they can't find the secret to reversing it, possibly having them being forced to seek out a cult leader of the elder evil in order to reverse it (which will create a really fun and difficult roleplay encounter). Or, when they hear the name, it summons an allip that the party has to fight/avoid once every so often (I would make it random to keep them on their toes, rolling some dice to determine when they show up). 

You could combine this with the short-term madness effect (I would just choose an option that fits the flavor) and really make the party squirm. Make a sense of urgency, keep them on edge, and don't be afraid to show them how crazy and otherworldly these entities are. They're dealing with creatures and powers completely alien to their world, and the more worried and paranoid they feel, the better.