You call your broker at GS and tell them what you want to trade and they go search for a counterparty, or potentially act as your counterparty. All very high touch and they're not going to do it for you unless it's going to be worth their while, plus since they're exposed to your counterparty risk you'll need to have a very large balance sheet that effectively no retail trader posesses. Basically if you have to ask how OTC works (and we're not talking pink sheets here) you haven't a chance of trading OTC. Not trying to be dismissive, just trying to save you wasting any more time unnecessarily.
If you're an acredited investor you may be able to purchase warrants or instruments like warrants which are somewhat like long dated options from those who were granted them for employment. Again it's a game you need to understand if you're going to play it.
If you haven't looked at traded warrants yet you might want to look into that. You can almost synthetically create a long-term option with one.
