Giroud struck in victory over Tottenham on Sunday.
"Considering the amount of speculation there has been about strikers coming in, Olivier has just got his head down and worked very hard. He is looking absolutely fantastic for us," said Walcott.
"You always have difficult patches and it's about the way you react to them and how you bounce back. Both Olivier and Aaron Ramsey were very good in pre-season and they both did very well against Spurs."
For the big stories and the banter follow us on Twitter: @tribalfootball