Wadham is absolutely chockablock with those on the far left. Its popularly known as the 'People's Republic of Wadham', even going so far as to rename one of their quads as the Ho Chi Minh Quad during the 1970s. Not to mention the whole anti-apartheid 'Free Nelson Mandela' thing, or Queer Week: One Wadham SU meeting is enough left-wing sillyness to last a lifetime.
Next is probably Somerville, who are particularly active in OULC (Wadhamites traditionally gravitate towards OUSU for some reason), although they have a habit of producing high-profile Tories like Maggie Thatcher. Balliol follows, known as a 'left-wing' college but more conservative than Wadham.
On the right, Oriel are usually regarded as demonic arch-conservatives, being the last all-male college to admit women in 1985 on the grounds that 'the distinctive character of the college will be undermined'. It has also severed its links with OUSU.
The rest of the colleges are somewhere in between. For instance, Keble and Mansfield are slightly left; St John's slightly right. Exceptions are the notoriously apathetic Turl Street colleges (Exeter, Jesus, Lincoln) who devote all their political fire to beating each other!