Drop down list with multiple values from a table depending on criteria

[jono123445]

=INDEX(O:O,MATCH(A4,Q:Q,0),1)

If the value of A4 (which has a drop down list with multiple possible values) matches the value of a cell in row Q, I need a drop down list with the corresponding values of row O.

With the above formula, I only get a drop down list with the first value that meets the criteria.

Is there a way of getting all values that meet the criteria to appear in the drop down list?

Many thanks in advance!