When you first start the app you'll be asked to download the base dictionary data - 450MB in total - which is best done over WiFi, while charging the device. After the download is complete 185,500 words, phrases and meanings are available offline. There's also a real audio recordings of each word words, spoken by a human voice, not a computer generated as is sometimes used in this style of dictionary.
With the main dictionary comes 83,000 collocations (words that go together) and 57,000 synonyms (opposite and related words via the integrated Thesaurus). There's also a further optional download of example sentences (a further 430MB download), though these can also be accessed online.
The app includes a powerful full dictionary search, which can be used to find any phrase or example sentence in the dictionary. Other features include the Oxford 3000 (keyword entries to learn the most important English words), Academic Word Lists, 1300 illustrations, bookmarks, and word information (usage notes, verb forms and word origins).
The app's My View settings let you choose how much information to shown on screen - it's possible to hide IPA, example sentences, pictures, synonyms and so on, which helps prevent information overload during casual usage (the full entry can be displayed wth a simple tap).
Marketplace description:
Improve your language skills with the Oxford Advanced Learner’s Dictionary. Understand what words mean. Learn how to say them. Know how to use them.
The Oxford Advanced Learner’s Dictionary (OALD) is a world best-seller and is now available as a Windows Phone app, with the full A-Z dictionary, real voice audio (not text to speech), and My View to customize your screen.
Oxford Advanced Learner's Dictionary (app name is abbreivated to OALD) can be downloaded from the Windows Phone Marketplace; a free trial is available with the full version costing £19.99 / $26.99 / €23.49.
That pricing obviously a lot more expensive than the average app, but is not uncommon for dictionary apps, which typically include a significant cost related to the licensing of the dictionary data. What you're paying for is the quality of the defintions, the completeness of the word list (includes up to date vocabulary such as malware, tweet, staycation and cloud computing) and the audio examples. If you are serious about learning English; or looking for a high quality, interactive dictionary then it is not an unreasonable price to pay.