While OUP is a commercial publisher, there are several legitimate, safe ways to access their dictionary content without a direct purchase:
Oxford University Press partners with to provide dictionary apps. Concise Oxford English Dict. - Apps on Google Play Concise Oxford English Dictionary 13th Edition Free Download
The story of "free" downloads for premium dictionaries often ends in a cautionary tale about cybersecurity. While OUP is a commercial publisher, there are
A truly free, safe, and complete PDF of the COED 13th edition does not exist legally. Any website claiming otherwise is likely breaking the law or trying to infect your device. While OUP is a commercial publisher
The Digital Lexicographical Shift: An Analysis of the Concise Oxford English Dictionary , 13th Edition