If you want the same look as Dulux Heritage Green Oxide out of the standard Dulux range, go straight for Dulux Paper Mint. It comes in at ΔE 0.2 from the original, which is well below the threshold where the eye can spot a difference at all — for practical purposes it's the same colour, just sitting in the cheaper, more widely stocked Dulux line. Its LRV is 62, so it's a properly light, airy green-grey that'll bounce plenty of light around a room.
Your backup is Dulux Weathered Glass at ΔE 1.9. That's still firmly in "very close" territory — anything under 2.5 is a match most people would happily live with. At LRV 64.5 it reads a touch lighter and slightly cooler than Paper Mint, so if you find Green Oxide a smidge too soft you might actually prefer it.
Now, the obvious question: why match at all when the Heritage version exists? Two reasons people do this. First, cost and availability — standard Dulux is mixed in more shops and tends to come in a bit cheaper. Second, sheen options: Dulux Heritage is its own paint system with its own finishes, and if you've already speced the rest of the room in standard Dulux Diamond Matt or similar, matching across keeps your finish consistent.
That said — and this matters — Dulux Heritage paint has a richer, more pigmented base than standard Dulux, so even with a perfect colour match the Heritage version will usually have a bit more depth and a flatter, chalkier finish. If the depth of the original is what drew you in, it's worth ordering a Paper Mint tester pot and living with it for a few days before you commit the whole job.
Get the tester on the actual wall, view it morning and evening, and check it against your trim. ΔE figures are brilliant for narrowing it down, but your own eye in your own light is the final word.