It is highly recommended that you choose the form that you believe most accurately represents your ability.
However, the OPIc is structured so that each form provides opportunities to test a range of skills and to provide opportunities to perform at the next higher level.
Additionally, you are rated not only on your ability to perform the task requested, but on the type of language you produce. For example, Intermediate level questions appear on all three forms. But a test taker can answer an Intermediate task like a Novice, like an Intermediate, or like an Advanced speaker.
For this same reason, you can not improve your rating by choosing an “easier” form