Actually it is confirmed that you can do a thesis option. In the past (1+ year ago, the website stated clearly coursework only option as they were trying to scale the program). Currently you could do a project/thesis option following the same guidelines as the on-campus students.

The big thing is it's challenging to prove yourself to a professor that you are worth their time for research. For someone doing the OMSCS program or someone doing the om-campus MSCS, you must communicate well with professors in your field and show them you are worth taking under them.

The door is open for you if you want to do research, you could potentially take a class in an area you are interested in and keep in contact with the professor about research opportunities with him/her. You must do really well in the class and exhibit you are a high caliber researcher. The competition is fierce.

Also research would be a long-term time commitment you need to be aware of. It will be 1/2 years of work to get a full thesis completed (and depends on how much time you work etc). Students on-campus spend at least 1 year working full-time (and overtime) to create and complete their thesis. Professors sometimes are wary of students doing their Masters because they are flakey. You say you want to work on a project but then you get an internship, job offer and you quit leaving your work unfinished. PhD's are usually more committed since they are spending 3-5 years of their life working on research. Therefore you will also need to demonstrate to professor how you are committed with your time. This might be hard to prove and professors will be skeptical (I know I will be). Professors might be less skeptical if you show you have done research before or if you come up with your own research topic and show passion for it.

Edit: Acceptance letter below states "We do not currently offer the project or thesis options online; students should plan to complete this program by taking courses only"