A reward of £50,000 for information leading to his arrest was announced in October, marking the two-year anniversary of Josh's death.
The 21-year-old from Kingsbury was stabbed in the neck in an unprovoked attack while partying at the Re: bar in Field End Road, in the early hours of October 11 2015.
Police believe O'Brien was flown out of the country on a private chartered plane from Biggin Hill airfield, shortly after Josh's murder.
Metropolitan Police has been tracking O'Brien, who was first believed to have travelled to the Netherlands and later understood to be in Dubai.
Detectives believe he has now travelled to Prague, where he was arrested in February 2017 for criminal damage and assault.
O'Brien had used his Italian alias, Enzo Mellonceli and provided Czech police with fake documentation to secure his bail.
An image taken by Czech Republic police shows he has grown his hair and has a full beard and a distinctive new tattoo on his back of an owl holding a skull, which covers up his previous “Shannon 15-04-06” tattoo.
He has also been known to use other aliases, including Enzo Mellonceli, Troy O'Brien, J Dunn and Glennister. O'Brien is one of 58 men on the Europol Most Wanted list, but one of just eight men for whom a reward has been offered.
Detective Chief Inspector Noel McHugh, of the Homicide and Major Crime Command, said that O’Brien had boxing gloves with him when arrested in Prague, and believes he is still using gyms in his hide-out.
He appealed for anyone who recognises him or has sparred with him, and hoped the distinctive new tattoo will help jog memories.
Police are also keen to hear from the artist who inked the new tattoo.
O’Brien is white, 6ft tall with grey eyes and dark brown hair. Anyone who sees O'Brien should not approach him but call police immediately on 999.
On October 28 2015 investigating officers arrested two men on an inbound flight on suspicion of perverting the course of justice and assisting an offender.
They were bailed to return on a date in October pending further enquiries.
Anyone with information about O’Brien or his whereabouts can call Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org or the police incident room on 020 8785 8099.
Other Brits on the list include convicted murderer Shaun Walmsley from Liverpool, who escaped from prison, and former Bangor City Football Club press officer David Daniel Hayes, accused of systematic sexual abuse of two children at their home.
In March, Roy Lawrence Piechocki from Hillingdon, one of Interpol's most wanted men in the world, was arrested in Bulgaria . Roy is accused of 10 counts of sexual exploitation of a minor in the United States.
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