Darklands Puzzle #30

A door blocks your path. An inscription reads:

Ooo, ooo, aaah? Is this some type of dwarf joke? In any case, five metal knobs are on the door, each bearing a number. Pressing the wrong knob may (will. ed.) spring a trap! You ponder, then press...
...0.
...2.
...4.
...6.
...8.

(Source: Darklands cluebook by MicroProse Software)

The leftmost digit, "A", must be one, as established in the solution to Darklands puzzle #2. The fact that the rightmost column, where "O" is added to itself, results in "H" in this instance, but that the same addition results in "A" in all other instances, indicates that carrying is taking place. The smallest number which, when added to itself, results in a carry operation is five. It is also the correct one. In other words, the decrypted addition is $555 + 555 = 1110$. This means that one must touch the knob labeled zero.