Adding late fees to existing tenants - what is legal?

[Kelsey P.]

I would love for someone to direct me to a resource (or answer my question directly here) to find out if I am within my legal parameters to add late fees if tenants do not pay rent on time. 

Quick background: My mom and I inherited several single family homes this year when my dad died quickly from brain cancer. He was a terrible landlord in terms of not having correct paperwork, procedures, etc. We have one house where the tenants act surprised when I ask them to pay rent - like wow - didn't know I needed do that this month. 

I know going forward when we place new tenants, we will have all the paperwork sorted out (I am meticulous and organized) and have them pay everything online (complete with late fees, evictions, etc.), yet I didn't know if I could legally have these inherited tenants start paying their rent online, require rent be paid on the first, and include late fees, etc. since we did not inherent them through a purchase. If my dad had a lease for them, it's just a few scribbles on a piece of paper with little to no official documentation. We had to physically go to the house several months back to get their phone numbers. 

The property is located in central Oklahoma in a super small town (about 1k). They have lived in the property for about two years. 

Thanks in advance!