"The keywords of the Oxford 3000 have been carefully selected by a group of language experts and experienced teachers as the words which should receive priority in vocabulary study because of their importance and usefulness. The selection is based on three criteria.
The words which occur most frequently in English are included, based on the information in the British National Corpus and the Oxford Corpus Collection. However, being frequent in the corpus alone is not enough for a word to qualify as a keyword: it may be that the word is used very frequently, but only in a narrowly defined area, such as newspapers or scientific articles. In order to avoid including these restricted words, we include as keywords only those words which are frequent across a range of different types of text. In other words, keywords are both frequent and used in a variety of contexts. In addition, the list includes some very important words which happen not to be used frequently, even though they are very familiar to most users of English. These include, for example, words for parts of the body, words used in travel, and words which are useful for explaining what you mean when you do not know the exact word for something. These words were identified by consulting a panel of over seventy experts in the fields of teaching and language study." (more description here)
Now Oxford has made available a profiler to complement the list. You can paste your text in and it will highlight in red any words that fall outside of the list. (By the way, Tom Cobb has a number of vocabulary profilers here.)