express (oneself) to (someone) on (someone or something)
To share one's thoughts on someone or something. Well, did you express yourself to David on that problem? He's not a mind-reader, you know. I never expressed myself to anyone on Laura—how did they know I have a crush on her?
express (oneself) to someone on something
to say what one thinks about something. I will express myself to Karen on that matter at another time. She expressed herself on Karen to the entire group.