Oceansize were an English rock band from Manchester, formed in 1998. The band consisted of Mike Vennart (vocals, guitar), Steve Durose (guitar, backing vocals), Richard "Gambler" Ingram (guitar, keyboards), Mark Heron (drums) and Jon Ellis (bass guitar) for the majority of its career, with Steve Hodson replacing Ellis on bass guitar in 2006.

The band released four studio albums, in addition to a number of EPs and singles, displaying a wide array of influences from several genres including post-rock, math rock, psychedelic rock, and space rock. Following a twelve-year career, Oceansize announced their split in February 2011, with the members moving on to different projects.

The band members met each other while attending music college in Salford, where the various members performed in a range of different musical acts very stylistically different from the band they would go on to create. "[The bands were] pretty terrible," said Mike Vennart. "We were in a grunge band. When I met Steve I asked him to be the new guitarist. I wanted to be experimental and unusual and still write pop songs. We were terrible at it. We didn't go anywhere for a while. When we got a new rhythm section, we re-thought what we were doing and got better. We had more vision. Although we don't all like the same music and have different tastes, we can see clearly where our music needs the most work."

The band formed after attending a Mogwai concert during their final year at University. Though known as a Manchester band, Vennart, Gambler and Ellis are originally from Yorkshire, Durose from Birmingham, and Mark Heron is Scottish. Steven Hodson, who joined the band later, is the only member of Oceansize to hail from the Manchester area.

The band subsequently named themselves after Jane's Addiction song, "Ocean Size". According to guitarist Gambler, the band's then-bassist Jon Ellis came up with the name: "I think, at the time, he was thinking what we would sound like. Jane's Nothing's Shocking album, which has the track "Ocean Size" on it, was definitely a big influence."

Over the next few years, Oceansize would release two self-released EPs Amputee and A Very Still Movement. This in turn led to interest from Beggars Banquet Records, who signed the band in 2000. The band's first release on the label was the Relapse EP titled after a very early composition that had been through many different reworkings.

Oceansize's debut album Effloresce was released on 29 September 2003 on Beggars Banquet. The album spawned the singles "Catalyst", "One Day All This Could Be Yours" and "Remember Where You Are", each of which featured a music video. The album garnered considerable critical acclaim, with music critic Ben Hogwood stating that: "with their broad harmonic language and fluctuating rhythms it's difficult to give an alternative to Oceansize, which is always a good sign. If pushed I would say they've taken a good liking to '70s rock but taken on board the works of bands such as Muse and The Cooper Temple Clause, along with the more expansive end of Seattle grunge music.".

Two years later, the band released their second album, Everyone Into Position on 19 September 2005. The album track "Meredith" was featured on the popular television drama The O.C., and one of the album's singles, "Music For A Nurse", became the soundtrack to an Orange advertising campaign entitled Fish during summer 2006. "Music For A Nurse" was also featured in the motion picture The Invisible (2007), and both "Music For A Nurse" and "Meredith" have also been used in the BBC drama series Waterloo Road.