No, a tab of Oxy's will not kill you. Not likely anyhow. Unless you're already frail in some way or another. Like, an underlying health condition or old age or some such.
No, you're more likely to ruin your liver with them. Maybe your kidneys in the process. You will survive and you wouldn't like what you made your life into by OD-ing on Oxy's any more than what you have now.
If you are absolutely certain you want to end your life... why not simply try gravity?
Really, find a high point... a bridge over a large gap. Doesn't have to be too high. Few dozens meters would do the trick. The more, the better obviously. And it is far less survivable than OD-ing on medication is. Far less survivable.
I would try to talk you out of it but you don't want that. So, I'm not going to try.
I am however going to suggest you do some research into what is and what is not fatal. Pick something you know will kill you. Not something where there might be a chance of your survival. Because if you survive, you will obviously have to suffer the consequences of your actions.
And you probably wouldn't want that.
Good luck and peace be with you if you should succeed. May your next life be more in line with what your soul is after.