A Statutory Plant Health Notice was issued on the trees on 24 July which requires the treatment and removal of the trees.
The infected tree in Roath Park Gardens was sprayed as a precaution using an insecticide last night and will be removed along with the other six trees on 25 July.
The Welsh Government said: "Our first priority is containment and clearance of the pest. We will now work with Natural Resources Wales (NRW) and Cardiff Council to ensure the trees are replaced.
"To prevent the risk of further spread a monitoring programme in the area will be conducted by NRW and Animal and Plant Health Agency colleagues with input from Forest Research and Welsh Government officials.
On 19 July 2019, we introduced tighter restrictions on the import of oak trees to protect native trees from the threat of OPM. Welsh Government is working closely with Defra, the Forestry Commission and other devolved administrations to ensure UK-wide protection against this pest.