Black Screen when Upgrading RAM in 2019 iMac
The 2019 iMac computer have different memory specifications depending on which model you get - the 21&quot; and 27&quot;; the 4K or 5K models.  Not all models support 128GB of RAM.
It's also important to note that OWC is not the original memory.  OWC is an after market brand.  If you're positive all of your memory works, but when you install the rest you get a black screen, you may be trying to install too much.  Confirm you have the right memory for your particular application.
The 21&quot; models officially support up to 32GB of RAM and the 27&quot; models 64GB.  However, independent testing per EveryMac.com confirms that they can support 64GB and 128GB of RAM for the 21&quot; and 27&quot; models respectively.

21: i3 4K iMac uses up to 64GB of PC4-19200 2400Mhz DDR4 RAM
21&quot; i5 &amp; i7 4K iMac uses up to 64GB of PC4-21300 266MHz DDR4 RAM
27&quot; i5 &amp; i9 5K iMac uses up to 128GB of PC4-21300 2666Mhz DDR4 RAM

You should be installing these in pairs, but there's no requirement to stagger the installation (slots 1 &amp; 3 or 2 &amp; 4).  See Install Memory in an iMac.  Memory is usually sold in &quot;kits.&quot;  A 32GB kit is two 16GB SO-DIMMs and a 64GB kit is two 32GB SO-DIMMs.  So, for example, for the 21&quot; iMac, you'll need two 32GB kits which will have four 16GB SO-DIMMS for a total of 64GB of RAM.
Here's a breakdown of the maximum memory per slot for each model:

21&quot; iMac → Max RAM per slot = 16GB
27&quot; iMac → Max RAM per slot = 32GB