OM is a big mod, and as such, would probably benefit quite a bit if you use the LAA (Large Address Aware) applet. There is a download on this site. CapnScurvy has an excellent write-up on it
Windows10; Large Address Aware; SH4 Install. While he talks a lot about installing and updating Windows 10 in there, there is more on LAA, and the different versions of that, and how to install and use it. All of which applies to all versions of Windows from Vista on, with very little changes. Windows XP is a different bird, but can also use a similar app, other than you only gain a few megabytes, since XP's addressing could only go to 4 gig anyway, and the OS eats up almost one gig by itself...
Also, mods themselves, no matter how efficient a modder might try to make them, can still impact the flow of the game. If you get certain parameters out of whack in a mod, you could crash the game by having the computer try to calculate too much...