Need to buy memory to install in iMac 21.5" late 2012, is the OWC brand good one to use?

Welcome to the Apple Support Communities.

You cannot upgrade the Memory on your iMac, only an Apple Authorized Service Provider (AASP). However, they might not since your iMac is vintage. Why do you want to upgrade the Memory? Is it due to performance issues? If so, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

rkaufmann87 wrote:

Also, your question is totally moot as a 2012 21.5" iMac uses RAM that is soldered in, this means it is NOT upgradeable.

Sorry, but that is not correct. A 2012 iMac uses SoDIMMs, they are not soldered in and can be upgraded just fine. It's just you need to disassemble the iMac to get to them.

Even Apple admits its upgradeable in the support document but that you need to contact an authorized service provider as an Apple Store won't do it.

 Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.

Apple themselves won't install any ram on a 21.5 iMac regardless of brand.

You will need to take it to an authorized service provider to get it installed as explained in footnote #3 in the support document you posted from.

Yes, macSales / OWC is highly recommended as is Crucial.com

See here: link-> https://eshop.macsales.com/shop/memory/iMac/2012_21.5/DDR3L

and here: link-> https://www.crucial.com/compatible-upgrade-for/apple/imac-(21*5-inch--late-2012)-imac13-1

Also, your question is totally moot as a 2012 21.5" iMac uses RAM that is soldered in, this means it is NOT upgradeable.

Finally, remember a 2012 iMac is not almost 10 years old and you may want to consider replacing it as it is obsolete and no upgrades are available for it. In addition, it has little resale value.

Best of luck.