The 68-year-old actor, who was engaged to the late Farrah Fawcett, was recently cast in the role of Teddy Montgomery's father.
"My dad comes in soon - they hired Ryan O'Neal to play my dad, as you know," Donovan told DS. "I was pretty starstruck when I had to work with him! I'm a big fan of his work so it was a big deal when he came on set.
"With this addition comes a whole new side of Teddy too, and a whole new dynamic between him, Silver and his dad. There's a lot of tension and drama there!"
Donovan also teased that O'Neal's character is "less of a father figure and more of a friend" to Teddy.
90210 continues Tuesdays at 8pm on The CW in the US and 9pm on E4 in the UK.
> Click here to read our full interview with Trevor