Morphine is not terribly active (1/6th) in a non user by mouth compared to IV, though it can certainly work. Using the dosages of morphine used for morphine only surgical anesthesia as well as converting the dosage of IM hydromorphone that can be used for lethal injections in Ohio, you'd be looking at around 2 grams oral morphine to start for a non user. I've chosen these numbers because there is a documented history with them being fatal. People,have died from much less, but in ctb, I wouldn't look for the absolute minimum number that might do the job. FWIW, the OxyContin originally came in 160mg pills that were discontinued because people kept dying after taking them. The equivalent dosage of morphine by mouth in a non user would be around 450mgs.
the cocktails in Oregon contain significantly more, as they are designed for rapid death and are often used by people with extreme tolerances. Oral opioids aren't very well favored here for some good (can be reversed, can take a while) and not so good (you might become addicted to them) reasons.