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You asked:

are siamangs old world monkeys

No. A siamang is not an Old World monkey.
siamang
siamang
siamang

Siamang, an arboreal, black furred gibbon native to the forests of Malaysia, Thailand, and Sumatra

Old World monkey
Old World monkey
Old World monkey

Old World monkeys or Cercopithecidae, a group of primates, falling in the superfamily Cercopithecoidea in the clade Catarrhini