Can frequent oral sex cause oral cancer?

YES - There are possibilities that Oral sex can cause Oral Cancer -

Oral sex linked to mouth cancer-

Some cases of mouth cancer could be caused by a virus contracted during oral sex, scientists have warned.Writing in New Scientist magazine, US researchers said the human papilloma virus, which causes most cervical cancers, may also cause oral tumours.

Though smoking or drinking causes most mouth cancers, but the HPV link could help explain why some young adults develop the rare disease. It is estimated that up to a fifth of women aged 18 to 22 in the UK carry a variety of HPV.

Scientists are working on a vaccine that would be effective against the most common strains in an attempt to cut the number of cases of cervical cancer.

They now hope a vaccine could have the added benefit of reducing oral cancers, which affect around one in 10,000 people. Most of those who carried the virus had the HPV16 strain - which is also the most common strain in people with cervical cancer.

The virus was equally likely to be present in men and women.
Patients with mouth cancer were also three times as likely to have antibodies against HPV, which shows they had been exposed to the virus, than the healthy people studied. The link was even stronger in people with tumours at the back of their mouths. -

Well I never heard this before, and when no one know the exact reason of oral cancer then you can't blame only oral sex for it.

No, oral sex in itself can not. What could cause oral cancer is any substance virus or bacteria you are exposed to while performing oral sex. Therefore, the bacteria or virus is the cause not the act of oral sex. Just don't be jumping from one partner to another and ensure you have a clean partner.