Like I said the OC wasn't established when Moses received the 10 commandments.
Hebrews 9:18-21, "Hence not even the first covenant was inaugurated without blood. For when every commandment had been told to all the people by Moses in accordance with the law, he took the blood of calves and goats, with water and scarlet wool and hyssop, and sprinkled both the scroll itself and all the people, saying, “This is the blood of the covenant that God has ordained for you.” And in the same way he sprinkled with the blood both the tent and all the vessels used in worship."
We also see in Exodus that besides the blood, the elders and other saw the God of Israel and they had a meal.
Exodus 24:9-11, "Then Moses and Aaron, Nadab, and Abihu, and seventy of the elders of Israel went up, and they saw the God of Israel. Under his feet there was something like a pavement of sapphire stone, like the very heaven for clearness. God did not lay his hand on the chief men of the people of Israel; also they beheld God, and they ate and drank."
What happened in the upper room. The there was the blood of the covenant, the disciples saw the God of Israel and there was a meal.