Oracle Conditional Insert Statement

Dear Experts,

I would like to insert data from one table to another with some conditions.

Think that we have table-A(a,b,c) and table-B(x,y,z). I will do  "insert into Table-A select x,y,z from Table_B" but with the conditions: if x='Sigma' the y will be x (the inserted value will be same as field x), if x='Teta' the y will be character value 'Beta' while inserting.  

Best Regards