This is an irregular verb. It follows the conjugation pattern of the verb pedir.
In European Portuguese, the present subjunctive, imperative, and negative imperative commonly use 'oiça' instead of the Brazilian form 'ouça'. However, 'ouça' is also used in some regions of Portugal, especially in the north.
Present Subjunctive:
Fale claramente para que todos o ouçam.
Fala claramente para que todos o oiçam.
(Speak clearly so that everyone can hear you.)
Imperative:
Ouça o que ela tem a dizer.
Oiça o que ela tem a dizer.
(Listen to what she has to say.)