Hello, and welcome!
The company you referenced mentions only FreeBSD on their website; FreeBSD and OpenBSD have not shared a common kernel since the final Berkeley code release in 1994 -- 4.4BSD Lite. These two projects may inspect each other's code and may import or derive functions by including portions each others' code, but that's it. There is absolutely no guarantee that what functions with one BSD may also function with another.
If your vendor is willing, you might ask them to test the proposed hardware package with OpenBSD and send you a dmesg, so you will know what works and what does not.
It's very hard to tell by looking at a product model number whether or not a particular driver will function with it. That's because the drivers communicate with underlying components -- chipsets -- which are subject to change at manufacturer whim, often without any product model number changes.
The official hardware compatibility list for the adm64 architecture is at
www.openbsd.org/amd64.html and is applicable for the most recent release, which is 5.1. However, the links to the driver man pages within it will point to -current man pages. And -current is already months beyond the next release, 5.2, which is in pre-order status and is expected to be announced and supported on or about November 1.
As one example of the complexity of this "will it work?" question, your hardware list includes an LSI/Dell RAID controller. The hardware compatibility page states that LSI/Dell MegaRAID SAS controllers are supported at 5.1-release, and it has a link to the -current
mfi(4) driver man page. In that man page, you will see that 9211 models are
not listed. That means that, at this time, that particular RAID controller is not listed among the supported models.
For each key component, you will need to conduct this research. Unfortunately, due to how manufacturers change underlying chipsets, model name alone is usually insufficient for surety. Better, always, is a test of the hardware.
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As for memory limitations, 96 GB should work. See this dmesg fragment that
rocket357 posted in April of last year, showing a 132GB amd64 system.
http://www.daemonforums.org/showthread.php?t=5859