The 24-year-old was picked after injuries to Jarrod Parker (Tommy John surgery) and A.J. Griffin (muscle strain in throwing arm).
"I was excited," Gray said about when Melvin told him the news. "I don't think it changes preparation. I knew I was going to pitch at some point. I'm looking forward to a bullpen [session] and my last start of the spring [Wednesday].
"It was something I hoped to one day get the opportunity to do, but to do it so early is special. Every starter kind of wants to have that opportunity."
Gray, who'll be Oakland's ninth Opening Day starter in as many seasons, went 5-3 with a 2.67 ERA in 12 appearances (10 starts) in his rookie season last year. He also made two postseason starts in the Division Series against the Tigers, allowing three runs in 13 innings with 12 strikeouts.
"He's very quickly become that guy for us — whether it was last year in the playoffs or being the Friday night guy at Vanderbilt, he was always that guy," Melvin said. "He's not afraid of the spotlight, not that anyone else couldn't do it as well. But we're comfortable with him.
"Not only the poise, but the competitiveness, how driven he is, and we've seen it all spring long here, too," said Melvin. "Whether he's throwing in a bullpen, pitching in a game, it doesn't matter to him, he's the same guy all the time and takes his 'A' game out there and is prepared every single time."