A 2-0 win over Wigan last weekend moved Sam Allardyce's side into the top half of the Premier League and onto 42 points.
The Hammers, promoted through the play-offs last year, would have been amongst the favourites for relegation but O'Brien reckons the goal was always much higher than mere survival.
"At the start of the season a top ten finish was always the target," the 27-year-old told West Ham TV.
"There wasn't any talk of relegation when we set out in pre-season and look at us now, we're right on track to finish in the top half of the table."
O'Brien, like many of his team-mates, had prior Premier League experience and the Republic of Ireland international is confident that has played a part in a successful campaign.
"As a newly promoted team that [a top-half finish] is a fantastic achievement," he added.
"Although Reading won the Championship last season, they've struggled this year. It's a hard league but we had players who had played in the top flight and that helped us from the start."
- Sports & Recreation
- Sam Allardyce