Australia are second in Group B of Asia qualifying, eight points behind runaway leaders Japan and equal on five points with Iraq and Oman with four matches remaining in their campaign. The top two from the group qualify for Brazil.
"Anything less than three points will be a disaster for us," the 40-year-old goalkeeper told reporters on Wednesday.
"It's going to be a very, very tough task for us."
Schwarzer, who plays for Fulham in the English Premier League, will lead the Socceroos in Tuesday's return match against the 105th-ranked Gulf nation in Sydney after being held to a 0-0 draw in the searing heat of Muscat last June.
They followed that draw with a shock loss away to Jordan in September, but the Holger Osieck-coached Socceroos put their World Cup qualifying campaign back on track with a gusty come-from-behind win away to Iraq, and are relieved to play three of their last four qualifiers on home soil.
"We've ended up being the team that plays three of the first of the four games away, and that was always going to be tough," added Schwarzer, who stands in for regular captain Lucas Neill, serving a one-match ban for drawing a yellow card during the Iraq qualifier.
"Obviously, if you look at where we're sitting on the table, it has worked out OK. We would have liked to pick up a couple more points along the way."
German Osieck, who has been under fire during 39th-ranked Australia's stodgy start to their final round of qualifying, named attacking midfielder Tim Cahill and a raft of Europe-based players in a strong squad for the Oman qualifier.