All graduate schools, including online grad schools, will at some point ask for a statement of purpose. They may refer to it as something else but they will all be asking for the same basic information.
Answer the basic questions of: What area do you want to study at graduate school and why? What is your current level of experience in your chosen field? What do you plan to do with your graduate degree?
As you answer these basic questions, demonstrate that you are dedicated to your chosen field of study and will be a diligent student. Be substantive rather than including "fluffy" language.
Don't write this as a personal essay like you did for your college admissions. Graduate schools are not as interested in well-rounded students as colleges are. Therefore, there's no need to talk about your hobbies, favorite undergraduate activities and such. Focus on experience that will support your pursuit of your graduate degree.
Write about your chosen graduate program excites you. Although the graduate school statement of purpose isn't a personal essay, it should be unique and reflect your personality. Discuss the passion you've developed for your field, this will help grad school admissions officers determine who will be successful students.
Finally, the statement of purpose is a reflection of you and your professionalism. Be sure to have it proofread by a trusted advisor for both content and grammar.
Don't use the same statement of purpose for each grad school application, tailor each to the particular school and program.
Carefully follow all the steps required on the graduate school application.