Count Orlo is one of the few characters on The Great who is explicitly based on a real-life figure. The real Catherine heavily relied on Count Grigory Orlov, just as the fictionalised Catherine trusts Orlo among her closest confidants and friends as she plans her coup. That's where the similarities end, though.
The real Orlov was a military man, not a scholar, and was a few years younger than Catherine, not older. In The Great, Catherine humourously attempts to seduce Orlo to win him to her side, but their actual relationship remains close but firmly platonic. That wasn't the case with the real Orlov and Catherine, as History points out: he was the first of her "favourites" and was her lover and partner for several years before and after her coup. They even had an illegitimate son born the year of the coup. Their romance came to an end some years into Catherine's reign, and Orlov lost much of his power and influence. Still, when he died in 1783, Catherine wrote in her personal papers about her intense grief.