Xiaobaitu, Pls Help!Can I use OPT? OR have to be EAD?

I filed I485 last September after I filed for OPT. Now I got my OPT and started to work with OPT. Is it legal? Do I need to apply for EAD and use EAD for work instead of OPT? Is there any potential trouble using OPT during the interview for 485 later?
I did not file EAD application yet just thinking if I can get green card soon then I may not have to spend that application fee.Thank you very much for your advice!