How can I address a cell in Excel using a text value from another cell?

I can't get my head around the following problem:

In two columns I have data. They represent 12 months of budget. Through a drop-down I am able to select a month and only the budget of this month will show in my second column.

Based on this selection I want to sum the budgets of the selected month and all the remaining months in that year.

With the MATCH function I retrieve the row of the budget of my selected month. Now I want to write my SUM function and I want to reference the cell of the selected month based on my MATCH result.

I tried ="O"&MATCH(A1;A1:A10;0) but that won't work as it displays text and is not a reference to the cell.

I know it is a simple problem and I need help.