Even if you avoid malware, pirated dictionaries are often corrupted or incomplete. A "cracked" version of the OALD 11 might load the interface but fail to load audio pronunciations (a key feature of the 11th edition). You might find that 20% of the new words are missing because the crack failed to patch the index correctly.
Often considered OALD's only rival, the Cambridge Advanced Learner's Dictionary is 100% free online with ads. While it doesn't have the 11th edition's "Speaking Tutor," its audio quality (UK and US) is superior. It also features the "Smart Thesaurus" and excellent grammar focus. oxford advanced learners dictionary 11th edition free