The singer-songwriter made the allegations in a strongly worded social media post, taking aim at the TV icon and her friendship with the disgraced filmmaker.
Sharing pictures of the pair together, Seal included the caption: ''When you have been part of the problem for decades... but suddenly they all think you are the solution. (sic)''
Seal, 54, described Oprah as ''sanctimonious'' and appeared to accuse her of knowing about the allegations facing the Hollywood producer.
He wrote on Instagram: ''Oh I forgot, that's right.....you'd heard the rumours but you had no idea he was actually serially assaulting young stary-eyed actresses who in turn had no idea what they were getting into. My bad. #SanctimoniousHollywood (sic)''
Oprah previously addressed the sexual assault scandal on her own Facebook page, describing Weinstein's behaviour as ''hideous''.
She said: ''I've been processing the accounts of Harvey Weinstein's hideous behaviour and haven't been able to find the words to articulate the magnitude of the situation.''
Oprah recently made world-wide headlines after a speech she gave at the Golden Globes sparked speculation that she would run for the US Presidency in the 2020 election.
In her powerful speech, Oprah spoke about press freedom, racism and sexual abuse before receiving a standing ovation with her closing words about the future.
She said: ''I want all the girls watching here, now, to know that a new day is on the horizon!
''And when that new day finally dawns, it will be because of a lot of magnificent women, many of whom are right here in this room tonight, and some pretty phenomenal men, fighting hard to make sure that they become the leaders who take us to the time when nobody ever has to say 'me too' again.''