The band infamously broke up in 2009, and rumours of them getting back together have been swirling ever since.
But speaking to NME, Liam said: "It is gonna happen, believe you me - it's gonna happen very f***ing soon because he's greedy and he loves money and he knows that it's got to happen soon or it won't happen."
The "he" in question is his older brother Noel, and Liam puts the urgency down to an offer of £100m he says the band received earlier this month to get back together.
Noel was quick to deny all knowledge of any such offer, brushing it off as a publicity stunt on the part of his younger sibling.
Liam, who insists the offer has been made and is currently being "booted around", puts Noel's response down to his "out of control ego".
Liam says the offer is for a tour rather than producing new music, but says he wouldn't be against making another Oasis record as long as it didn't sound anything like Noel's current band, High Flying Birds.
He says it's an embarrassment the rock band haven't already sold out their two-day stint at the O2 Apollo in Manchester in March.
But despite the ill-feeling, Liam says he's bored of being a solo artist and misses the camaraderie.
The 47-year-old said: "Doing it with our kid and with Bonehead [Oasis guitarist Paul Arthurs] and the lads years ago was what I was into, was how I envisioned it.
"Being in a band - it was about the Roses, it was about The Beatles or The Stones or The Pistols."
He goes on to compare himself to Rick Astley or Julio Inglesias, saying he's "gutted" to be on his own.
Gallagher has two solo albums, As You Were which came out in 2017 and Why Me? Why Not? which was released in 2019.
Both reached number one in the albums charts.
Next year, the father-of-four will tie the knot for a third time, marrying girlfriend Debbie Gwyther in Italy, with the third Gallagher sibling, Paul, as his best man.
Liam previously married actress Patsy Kensit in 1997 and All Saints singer Nicole Appleton in 2008.
It's not clear if Noel will attend the ceremony, but he has been invited (on the stipulation of their mother, Peggy).
Liam did not attend his brother's wedding to Sara MacDonald back in 2011.
Liam Gallagher will be touring the UK from 12 June.