You have two entirely different tickets and neither carrier even knows about the other. But, nonrefundable does not mean non-changeable.

As to the OZ ticket, consider that if your flight is changed/cancelled, you are entitled to a full refund without penalty. You may then purchase whatever you want. Might there be something cheaper or not much more which gets you to IST on another carrier, perhaps in a less convenient manner? Why not start with that research.

As to the TK tickets, are they changeable for a penalty? If so, how much? Sometimes this can be a lot of worry about not that much. But, do not skip any segment and "join" your ticket later. As soon as you skip a segment, the remainder of the ticket will be cancelled.

Finally, take a close look at your travel insurance. While few policies cover this situation because you chose to purchase separate tickets, some may provide some assistance, particularly if there are costs associated with the OZ changes. Maybe some of your losses, if any, can be made up in that manner.

As to what you do and do not want to hear, bear in mind that this advice is not only for you. There are others considering booking travel involving more than one ticket. It can lead to considerable savings at first glance. But, it also carries considerable risk. Pointing that out is entirely appropriate.