Hi folks this issue has caused me to join up and see if I can help spur anyone into solving this very common scenario.
First of all keep your saved world!!! you haven't lost anything yet it is all still there. Save a version off to a different part of your hard drive before you do anything in case it is solved at a later date.
I was half way and two months into a very pleasing Death Star model when this happened to me so I am keen to see it resolved.
Depending on what you want to retrieve , you can copy a version of your world into minecraft proper and it should load but you will lose any bricks that are not available in the un modded version( don' forget to back it up before this ). Then exit and copy it back to your tekkit saves folder and you should at least be able to open it. This was no good to me as most of my model was build from Galacticraft stuff.
This caused me to look behind Galacticraft to see how it was put together and I came across the class files that run the thing. I opened some these up in to see how I might trick the world into at least loading with minimal loss. Unfortunately I'm into graphics not programming and so can do little but it did cause me to think along these lines...
Within the Galacticraft download is a bunch of class files that tell minecraft what everything is and how it behaves and in most cases they are separate files and in some cases two or three.
The file(s) that concern the oxygen sealer are there and editable for someone that knows how it works. In theory you could create a version of Galacticraft that loads everything but the oxygen sealers to prevent the dreaded loop that crashes a world, just as Minecraft original does. this would be a small loss and very easily replaced once the world is reclaimed with the main structures intact. You could then replace this version of Galacticraft with the original and play on. Galacticraft is a great mod for us explorers and it is a real shame to have such issues with something as necessary as oxygen.
Here's hoping...
