Oi, mush! (or oi, moosh) is an aggressive way of calling out to someone -- like 'hey, you!'. The OED says that mush probably comes from the Angloromani (the Romany language spoken in England) word for a man, mush. The first thing that struck me was how similar it was to Russian man-related words: muzh (муж), husband; muzhchina (мужчина), man; and muzhskoi (мужской), male or masculine. The zh sound in all of those words sounds more like 'sh'.
Romany (or Romani) is the language of gypsies (actually it is a lot of different languages, or dialects, and there is no standardised version) and, like Russian (and English), is an Indo-European language. It is similar in vocabulary and structure to some of the languages spoken in the north and the central part of the Indian subcontinent, such as Punjabi.
Oi, mush! is Cockney slang, and other Cockney expressions are believed to have a Romany origin, probably because Romanies settled in the East End in the 19th century. Dukes (or dooks) meaning 'hands' (as in 'put up your dukes') is from the Romany word for fortune-telling or palmistry, dookin/ dookering. That is not mentioned in the OED, but 'pal' being from the Romany word for 'brother' is. Other words that the OED believes are from Angloromani are the fairly modern derogatory term chav (from the word for young man, or boy), nark (an informer), perhaps from the Romany nok, nose, cushty, slang for 'very good' (and from the Romany word for 'good') and wonga, slang for 'money'.