Recommendations from officers that know you (O-3 and higher) should hold more weight than a basic letter from the President, unless the Pres knows you personally. Of course if you know him personally, have him call the Sec Def to call the board and get you a slot automatically :icon_tong
I worked for Senator Bob Graham's office (D-FL), and made my first application to OCS while there. I received a basic, he's a good guy, letter signed by my boss (the Constituent Liaison) instead of the Senator. I even was doing Charlie's job while he was on vacation, calling the Pentagon and VA for people, and only received a basic letter. Calling the big house was kind of cool, though...
Anyway, I digress. Have a couple department heads and Div-O's of O-3 and above do some write-ups for you, and ask to get an interview with at least one higher ranking officer (O-5 and above) in the areas you would lie eto apply.
My input, take it as you will.