Given two .ass files, it creates a new file with the OP/ED lines of the first one replaced by the respective lines of the second one.

oped.rb

#! /usr/bin/env ruby
abort "usage: [ruby] #{File.basename $0} <ass> <op/ed>" if ARGV.length != 2

source = File.read(ARGV[0]).split(/\n/).reject(&:empty?)
lyrics = File.read(ARGV[1]).split(/\n/).reject { |line|
  tag = line.split(?,)[3]
  tag != 'OP' && tag != 'ED'
}

File.open("#{ARGV[0].split(?.)[0..-2].join}_OP_ED.ass", ?w) { |ass|
  source.each { |line|
    source_tag = line.split(?,)[3]
    lyrics_tag = lyrics.first.split(?,)[3] if lyrics.any?

    if source_tag == lyrics_tag
      timing = line.split(?,)[0..8].join ?,
      text   = lyrics.shift.split(?,)[9..-1].join ?,
      line   = "#{timing},#{text}"
    end
    
    ass.puts line
  }
}