Yes you can and i did.

I got the 2019 27 inch model with 128GB Ram installed, MAX OUT! but you can only use the compatible models and tested by users or run the risk of crashes. I will save you the research time, this is what i got and with shipping + import, it's still cheapest than what you can find in Malaysia. In fact, i don't know anyone in Malaysia carried anything with large memory sizes for iMac. Even if they do, it's VERY EXPENSIVE. I asked one vendor with half the size and twice the price! Please note than there are few brands sell compatible memory for iMac, cheapest you can find still Amazon but not all can be shipped to Malaysia. Also, few vendors selling the same model as well with different pricing but not all will ship to Malaysia. You just need to fine one that does.

If you get it from amazon, it will arrive about the same time you order iMac from Apple online - fast!

This is what i got.
https://www.amazon.com/gp/product/B07N124XD...0?ie=UTF8&psc=1

The reviewer that has trouble with Mac Mini said it has updated to Catalina and still have trouble with booting. So it wasn't Catalina that has compatible issue with his unit. If that's the case, you can easily google Catalina and samsung memory on Imac, you would find plenty of complaints. His probably have a bad memory, that's not unusual, just bad luck. Memory compatibility has little to do with OS and more to do with hardware.

I don't usually upgrade OS. i used it for DB, programming and mobile developing apps so OS compatibility on my apps is very crucial. if it's working, don't be a smartass and upgrade OS that doesn't benefit your workflow - that's my thinking. I also have a MBP and it's still running High Sierra. I don't plan to update my iMac unless having issue with applications for a year or two. I used mine for work along with photos and video editing. 128Gb is just right, i have seen my memory usage peaked 90+GB, 128GB is not overkilled for me. you can try 2 sticks of 32GB then upgrade when needed.

Amazon is so successful not just because the price is competitive, the return is top notch as well. It's the BEST in the World! Ok, may be 2nd to Cruthfield. A case when i was in US ordered a next day purchase and the package came in a day late, called in to complain and they told me to keep the entire package for free, not just returning the difference in carrier fees but the product as well! Well, It wasn't a valuable product but $40 USD nevertheless.

Obviously, returning product oversea can be difficult, costly and a lengthy process but i believe Amazon international order return policy is they will bare an x-amount of return fees in case of defective product. $20 or $25, somewhere there. So if you need to return in express service, then you need to shelf out your own cash.

The process is easy, just go to your order, click on return and print out shipping label and check reason to return. I use both Amazon Japan and Amazon US. The processes are the same.

This post has been edited by zeroendless: Jan 6 2020, 03:56 PM

I think 128Gb is the max for iMac 27. There's no vendors selling 64GB stick, OWC or not.