An open educational resource is one made available under a relatively unrestricted licence. The most common suite of licences for content (text and media resources) is the range of Creative Commons licences. These offer a number of options as to whether reuse must be attributed, whether use is restricted to non-commercial purposes, and whether derivatives have to be relicensed under a similar licence. There is no provision under Creative Commons to restrict reuse to a particular sector or a particular activity (such as 'educational use'). It is therefore not possible to release materials under a Creative Commons licence with an 'educational use only' restriction. It is possible, however, to release materials on exactly the same terms as a Creative Commons licence with the addition of an 'educational use only' restriction. Although this may seem the same, the additional restriction means that the resource cannot be said to be Creative Commons licensed, and it is less clear that this would be an 'open' educational resource.
The terms ‘educational use’ and ‘educational establishment’ are contained in the Copyright, Designs and Patents Act 1988 (CDPA), in relation to certain exceptions to copyright infringement. Fair dealing under the CDPA is unlikely to succeed as a defence when copying texts and uploading them to the open internet. Even where materials are used for purposes of non-commercial research, private study, criticism and review, and for educational purposes, making text freely available on the internet may be considered making multiple copies of the material and could still be considered a breach of copyright. Without a licence, you should contact/trace the copyright owner to seek permission directly. This may be a timely and costly exercise, however.
To make materials freely available to all, your institution should perhaps firstly consider using works where the copyright period has expired (for literary works (the life time of the author plus a period of 70 years from the end of the year in which the author dies). This would allow you to use the material without restriction. Secondly, if you are uploading content that is owned by the university (e.g. where a tutor creates content this is generally owned by the employer), you will be able to use it, giving due regard to any restrictions provided by your institution. Where OERs are made available online and include materials copied from third parties, even where the materials are deemed to be for educational purposes, permission is likely to be required either directly, or by way of a licence.