Travaux préparatoires (TP) is short hand for the documents used during the negotiation and drafting of a treaty. Vienna Convention on the Law of Treaties, Article 32 allows that TP can be a supplementary means of interpretation in certain instances. The preference is that terms in a treaty should be understood as having their ordinary meaning.
In some cases, there is a publication - or in the case of Human Rights treaties a freely accessible database (see link below) - which has done all or most of the hard work in gathering these together.
To discover what are available in Oxford (some examples in box to left), search SOLO with the phrase travaux preparatoires anywhere in the record.
If there is not one for the treaty you are interested in, please read the online guide via link below to make sure you have realistic expectations of what might be possible!