OMM has 2 meanings
On my momma
OMM means on my momma, as in "I swear on my momma's life." It is used to emphasize the truth of your claim.
OMM is similar to saying "I swear to God," which is only used when you really want someone to believe you. Since God and moms are pretty much the "holiest" things in the world, you better not be lying.
OMM, I have never been to a concert in my life.
|ISTG||I swear to God|
|SOOF||Swear on our friendship|
|blvmot||Believe me on that|
|BION||Believe it or not|
|COT||Circle of trust|
|TBH||To be honest|
People who are thinking about something use OMM to mean "on my mind." Generally, people use OMM when they are thinking about either one very specific thing or many competing concerns.
If someone tells you they have something on their mind, you can use W^, sup, or wup to try to get them to talk about it. If the person doesn't want to talk about it, you're probably best off letting them continue to ponder on their own.