Es gibt eine Vielzahl an Verwendungen für beide Präpositionen:
On
Used to express a surface of something:
I put an egg on the kitchen table.
The paper is on my desk.
Used to specify days and dates:
The garbage truck comes on Wednesdays.
I was born on the 14th day of June in 1988.
Used to indicate a device or machine, such as a phone or computer:
He is on the phone right now.
She has been on the computer since this morning.
My favorite movie will be on TV tonight.
Used to indicate a part of the body:
The stick hit me on my shoulder.
He kissed me on my cheek.
I wear a ring on my finger.
Used to indicate the state of something:
Everything in this store is on sale.
The building is on fire.
At
Used to point out specific time:
I will meet you at 12 p.m.
The bus will stop here at 5:45 p.m.
Used to indicate a place:
There is a party at the club house.
There were hundreds of people at the park.
We saw a baseball game at the stadium.
Used to indicate an email address:
Please email me at abc@defg.com.
Used to indicate an activity:
He laughed at my acting.
I am good at drawing a portrait.
eine einheitliche Regel gibt es leider nicht.
LG