As far as I can tell no. Does this mean you could have an evil oracle of a good god? I think that would be a cool idea for a villain. Maybe he would just be a follower "in name only", and would pervert his deities teachings to serve his own evil ends.
No, they do not have alignment requirements like a cleric. Their divine power is more often than not something that happens to them, rather than being granted by a particular deity. They can worship a deity, like any character, but it has no impact on their spellcasting.
I remember once designing a PC whose family was cursed by Lamashtu. She was thus an oracle of Lamashtu who absolutely hated Lamashtu.
Not only do Oracles not have an alignment restriction, they don’t even have a requirement to follow a deity. An Oracles power is granted to them by their mystery which is often a group of deities instead of a singular deity. In some cases the oracle can even be opposed to some of the deities supporting the mystery. Some of the mysteries have deities that normally oppose each other as sponsors of the mystery. Flame lists both Asmodeous and Sarenrae as deities.
An Oracle does not choose to be an Oracle, they are chosen by the powers supporting the mystery. In some cases they may decide to become a follower of one or more of the sponsors, but they could just as easily work against the sponsor.