“Windows is designed to be a very locked down OS”. Hmm, Windows is one of the all time least locked down OS’s. Granted W10 is better and I’ve not taken much of a look at W8-10 security as I’ve lost interest, so it’s possible that pigs fly and all that. (Just saying I’m out of touch, but would be utterly amazed.)
In other words I have a hard time thinking that W10 can be more secure than any BSD. Microsoft once received a (government rated) security rating by not having any network or external device on it. It was the only way to get it as any connection would allow remote access to an OS that simply could not be locked down sufficiently to prevent remote access and control.
Unix was developed by AT&T who wanted to be able to run their phone system reliably. All the BSDs and Linux are developed on the same principles and are inherently more secure and easier to secure. As an example an OS has rings of security where the core is running in ring 0. Nothing external should be allowed access to ring 0 as that would open the door to full control. Windows however had drivers running in ring 0 causing a huge hole that could not be plugged by any effort. Users were running with admin access by default where the Unix based had that separated from the get go.
“… XP requires far less RAM compared to BSD if BSD can run on older computers?”
W95 was bridging 16-bit and 32-bit technology and based on DOS which is such an old technology that only the most fringe developers even support today. I used to have a computer that ran on 4K of RAM, which preceeds all other single board computers. But it would be silly to bring in (either) as a comparison with modern OS’s unless you are not looking at a desktop solution, which you are yet to indicate.
If you have an old IBM PC, or similar, that you want to run a modern OS on you are going to have a hard time finding much luck. This thread really needs some specificity about your intended use. (To be honest I even considered this to be a troll, but simply not having much understanding can lead to the same impression and I like giving people the benefit of doubt.)
You could always get hold of OS version of the day (1995 era) and run those. I had a Linux server which had 8 MB RAM and ran a number of things at the same time, but I would never consider going back to that now.
How OpenBSD or any other works is not very relevant, from a user perspective. You install the OS of choice and use it via the desktop environment it provides. With Unix based OS’s you can install very different desktop environments that has different features and look and feel.
Get a different hard drive and install it in place of the old one, to keep what you have safe. Then install different OS’s and see what you like. As noted above TrueOS is aiming to give a nice user experience of FreeBSD, a very old and stable OS.