The full form of OP is: ORIGINAL POSTER

What does OP mean in reference to Snapchat?
OP is a common in the world of social media or online forums and stands for original poster. This term usually refers to the person who originally posted a content, wrote a thread or wrote a question.
If the content goes viral and is transferred across various platforms it becomes difficult to track who started the conversation. Hence the person who started the trend can declare himself the OP of the viral post/content.
Why did the term OP emerge? Why is tracking of OP important?
The term OP is generally used for the original producer of the content and has been used for many years now. If the feed becomes viral then the OP can get recognition or fame because of it.
Tracking the OP can also help in situations where the post leads to unrest in a large section of people. This can help the authorities to track down these posters and hence condemn for the riots, civil unrest, disrespecting organization etc.
This helps in curbing bullying on social media platforms and also keeps a tab on people who want to propagandize there agenda in negative fashion.
What are the examples of use cases of OP in sentences?
Following is an example of how OP is used in real life:
Person A: I don’t think I am off topic to this conversation and I am sure the OP agrees the same.
Person B: I think your views are not relevant to what the OP is trying to convey.