According to its website, the multi-disciplinary contractor has 70 employees, whose jobs are now in jeopardy.
Current Opco projects include the £6m refurbishment of Swansea's Glynn Vivian Art Gallery.
Opco was established in 2001 and has worked across south Wales, the southwest and the southeast.
The company saw its turnover jump from £23m in 2010 to £30m in 2011 although pre-tax profit shrank that year from £711,000 to £117,000. The 2011 accounts showed £2m cash in hand.
The administration is being handled by the Reading office of KPMG.