Suppose you could selectively prevent production of a-amylase or oligo-1,6-glucosidase in an organism that normally hydrolyzes starch. which enzyme would the organism miss the most

The enzyme the organism will miss most is ALPHA AMYLASE.
This is because, alpha amylase is the enzyme that is responsible for hydrolyzing alpha bond of starch in order to break it down to maltose. Oligo 1,6 glucosidase on the other hand is only able to hydrolyses alpha 1,6 glucosidic bond, which is just a step in the conversion of a polysacharride to smaller sugars.