I did the same (police report) in 2017 on Visa exempt stay . As I told them (police) in 2018 I will stay on OA visa they suggested it's better to see Surat Thani immigrations. So I went there did my TM30 and got my receipt.
Robert Lagas as Matthew answered already, just save it and you're done. You can always retrieve it to check it's stored in the system. For evidence, if needed, do a screen print.
calculated correctly 17.Feb.2019. As Chiang Mai ist hard core about TM30 ( reporting address within 24 hours after arrival ) make sure that your landlord has done this and get a confirmation.
Daniel Wolfe don't brood,get the O-A as James Miller stated , follow the rules and enjoy up to 2 years stress-free living in Thailand. Then decide yearly extensions or start again O-A.