The Czech Republic captain said he did not expect to see his fellow Gunner make Roy Hodgson's final 23, and thinks Oxlade-Chamberlain has the potential to make a huge impact on the tournament.
"I was surprised to see him in the England squad, but he definitely has the tools to succeed," Rosicky told Eurosport. "When he comes on as a sub, he is a real impact player.
"He can change the game, and brings something very fresh. I'm really happy for him, and it will be a great experience for him going straight into the Euros. Potentially, he has a huge upside and can develop into a great player."
Rosicky backed another Arsenal winger for a starring role for England, saying Theo Walcott is ready to start for England in Poland and Ukraine.
He said: "Theo is a proven player now. He is a nightmare for any left-back, and he can make a consistent impact. I definitely think he should be in the starting line-up for England."
Rosicky goes into the tournament on the back of an excellent season for Arsenal in which he finally shook off his miserable injury record.
He said: "Maybe it is my best time since I was at Arsenal, because I was quite happy with the second half of the season. From January, we started to play some great football.
"I am confident and happy in my own form. I have been fit, and the manager has been picking me consistently so that's very positive."
The 31-year-old, who misses this weekend's friendly against Israel due to a calf complaint, is desperate to taste major tournament action again after injury forced him out of Euro 2008.
"For me personally, it's great to be back at the Euros after missing 2008 through injury - it definitely gave me some extra motivation to qualify again, because I have not played at a big tournament since the World Cup in Germany."
Rosicky has eyed a key role for in-form goalkeeper Petr Cech, whose saves helped Chelsea to an FA Cup and Champions League double.
He said: "I hope it gives him a boost because we will need him, that's for sure! He played great against Bayern, and that's good news for us. He will be relaxed and happy."
The Czech Republic play their Group A games in the Polish city of Wroclaw, just 60 miles from the Czech border - the ideal outcome, according to their skipper.
"We wanted to play in Poland and we got our wish," Rosicky said. "All three games are in Wroclaw, which is the closest city to the Czech Republic, so it's a perfect situation for us. It's good for our fans, and hopefully they will come in numbers.
"We certainly have a good enough team to qualify from the group. We did a good job in qualifying and the play-off against Montenegro, so we deserve to be there.
"Our group doesn't sound exciting, but it will be very even and I'm sure it will go to the very end. Russia and Greece both won their groups in qualifying, and Poland are hosts, so that will be a proper away game."