Formula IF or Switch with multiple values

Hello,

I have a table with fields like this :

The "First char" field is a formula that return the first character UPPER of the "First name" field.

Then, the field "check" is used to filter a view from A to E. So i need a formula that show "OK" if the "First char" field contain A or B or C,,,,,,,,,E or else it will show "KO"

Any ideas how to do it ?

Thanks.