I am getting the sweet syrup smell when I turn the heat on for the first few minutes. I had this a few years ago and replaced my heater core and it was gone.

I have read that this could be caused by a bad/leaking heater core, or possibly the 2 O rings could be leaking.

I don't notice any fluid on/under the carpet. I was thinking of trying to replace the O rings before I get a new heater core (cheaper solution). Can anybody share the process of replacing the O rings? Does this make sense how I am going about this?

Thank you in advance.

I replaced my heater core a second time after about 18 months with the first one. It seems to me if the o rings fail, you will see it externally, so I suspect it is the core. Both times I could see just a little antifreeze color at the top of one of the tubes. Just a tiny leak but most noses can detect a few parts per million of ethylene glycol. In the 1997 850 it was fairly easy to do. Make sure you blow out the coolant from the engine side of the firewall AND put down plastic and then lots of towels after pulling back the carpet.
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1993 240 Classic; 1992 240; 1997 850 GLT; Mitsubishi 4 cyl.

was it a OEM heater core or an aftermarket?

I'm guessing you have to remove the heater core to replace the O rings anyway.