Onllwyn, along with nearby Banwen was a former coal mining community which had hundreds of men working in five local mines. One of its pubs, now demolished, had what was claimed to be the longest bar in Britain. This was needed to accommodate all the miners when they finished work and came off-shift. Close by there is still a well concealed coal processing plant and washery in operation.

The village has a community college, village shop, post office and pub. Close by are the Dove workshops and the village rugby team is still active and enjoying good support.

The Roman road Sarn Helen ran through the village and some parts are still evident. There are also the remains of two Roman forts within the parish and the Roman connection links back to Saint Patrick.

Legend has it that St Patrick, the patron saint of Ireland was born in nearby Banwen in 385 AD. His Welsh name was said to be Maewyn Succat and that his father Caiphurnius was a Roman official based at the Roman marching fort at nearby Coelbren. Maewyn was kidnapped at the age of 16 and taken to Ireland as a slave. After six years in Ireland he escaped to France and entered St Martin's monastery in Tours where, he became a priest. The Pope named him Patricius and sent him on a mission to Ireland in 431AD where he died on March 17, 461 AD.

There is a plaque in Banwen which tells of the link between the Irish saint and the village.