如何根据Libreoffice Calc中另一个单元格中的值更改地址?

I need to pull data from the source spreadsheet  $Units_Weekly, from a changing column.
The start is column **O**  ----> $Units_Weekly.$**O**2
Column name changes in A50. The next column name will be **P**,
it's based on a condition. 

How it is possible to link A50 and $Units_Weekly.$O2? 
I mean to insert content of A50 in to the formula above?
I have tried to use direct substitution but that does not work. 
This gives error ====>  $Units_Weekly.$&"A50&"2