Just got back from the trip.
Being OZ Diamond *G, OZ check in people were very nice. Checked baggage all the way through OZ-OZ-TK even if TK was on a separate PNR. Baggaged tagged priority and offered me two lounge passes without me even asking for them.
Checking in PRG was a very different experience. Our route was TK-TK(OZ codeshare)-OZ with the first TK leg on a separate PNR. There was a "supervisor" overseeing the person at the counter, but this was one of my worst check-in experience. After stating my connecting TK flight number, it seems all I said in English just missed her. I gave a copy of my itinerary written on my phone, the "supervisor" said it is an OZ-coded flight and refused to check in. I told her it is a Turkish Airlines plane! She proceeded to tell me without checking in their computer that OZ is entitled to a few seats on that TK plane but they cannot help me check in because it is OZ will handle the check in! I was incredulous that she will say such things as there will be no OZ counter in Istanbul as they did not have any flights to/from Istanbul that day! But just decided to give up, and she said we have to clear immigration and customs in Istanbul and check in back again ourselves. We do have Turkish electronic visas prepared for this scenario, but they did not ask for such documents (which should be their job) and just checked us in up to Istanbul only.
The problem did not end there though. I was traveling with family and there were 4 of us. We had 4 suitcases checked in, but the person tagging the bags did not put them altogether. She checked it in one per passenger and stuck the tags, one each to each person's boarding pass. And since she did not go through with us again (as is the procedure in most cases), it was not until we passed passport control that we noticed we only had 3 baggage tags and missing one!
I asked help from PRG information airside but she said she can't even contact the TK check-in desk. So I have to go through immigration again and go back to the check-in counter! When I explained the situation to the "supervisor", she came back and wrote the baggage tag number on a boarding pass. She said it was checked in, and the person tagging said she was sure she stuck the tag there on the boarding pass and I must have done something that it fell off! It is now my fault again! Anyway, did not want to argue with her, so went pass immigration control again. At least I have baggage tag information to trace the bag.
Upon arriving Istanbul and checking with the TK transfer counter, the agent there actually asked for our baggage tags and said they will transfer and retag our baggage themselves, so we do not have to pass through passport control! She gave us new baggage tags. The bag with only information written only on the boarding pass managed to be retagged and made it to our final destination, so it was such a relief!
One thing I have learned through this ordeal is to make sure one has a printed plane ticket for any travel using codeshares. Since I usually check in online, I usually do not bother anymore. But I found out the hard way that one cannot usually check in online if one is on a codeshare ticket. And the English skills of TK ground staff in PRG and IST not being very good, they seemed to be not able to locate our flights that well. I told the agent at IST that we are on TK 88 going to Seoul, Korea. Gave her additional information but she seemed to have difficulty until the one seated beside her said something in Turkish, with the only thing I understand is Incheon! That seemed to make her able to locate our PNR.