The last ability will trigger whenever you put any kind of sticker on this enchantment, not just name stickers, although only o’s in name stickers will count towards the amount of damage dealt. Notably, o’s in ability and art stickers won’t count.

Count the number of o’s in name stickers on this enchantment as the last ability resolves to determine how much damage is dealt. If this enchantment is no longer on the battlefield at that time, use the number of o’s its name stickers had when it was last on the battlefield.