The majority of Alex Neil’s squad returned for pre-season training this week, 32 days after beating Wycombe in the League One play-off final at Wembley.
“Pre-season has come around very quickly – it only feels like yesterday that we were lifting that trophy,” O’Nien told the club’s website.
“I’ve probably watched the goals at least 100 times, and have been lying in bed looking through the photos.
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“But now that’s in the past. We have things we need to go out and achieve – everyone came in early today to catch up and we had a few tests. I think the volume will get cranked up day by day now – it’s nice to see the boys again, and we all know what’s in front of us.”
“We’re excited for Portugal,” O’Nien added. “It’ll be nice to go away and train with the boys, grow closer to one another and we’re going out there for the job of getting fitter.
“Last time we went out there, we came back in good shape, played some good teams and this is going to be no different. We have the focus of coming back a better team."
When they return to Wearside, Sunderland will play four more friendly matches away from home, against Dundee United, Bradford, Accrington Stanley and Hartlepool, before their Championship opener against Coventry on Sunday, July 31.