Diminished Concepts
Over a G7, functioning as the V chord in the key of C, you can play these diminished scales to go out. Notice each one gives a slightly different feel.
G Half Whole Diminished 1-e2/e9-#2/#9-3-#4/e5-5-6-e7 this would be the most common.
G Whole Half Diminished 1-2-#2/#9-3-4-#4/e5-#5-6-7
G# Half Whole Diminished e2/e9-2-3-4-5-#5-e7-7
G# Whole Half Diminished e2/e9-#2/#9-3-#4/e5-5-6-e7-8
Therefore you can play these diminished scales over any dominant chord as well as any dominant's ii chord. Practice these formulas over a ii V to get use to their "feel" or sound.
Practice playing in key and then out using each of these diminished concepts.