Spokesman Bruce Randall said: "We have got to check the structural safety before we can do anything. It could be a matter of someone [one of our engineers] climbing up the mast, which could still be hot or could be structurally unsafe, and we are not going to consider doing that until we know it is safe.
"We would like to apologise, but these are highly unusual, difficult circumstances and we would like for people to bear with us. We will move channels back on to analogue if we can.
"Sometimes these things can take a matter of days rather than hours to fix."
About 400,000 homes have been left without television reception after an "intense fire".
Firefighters said part of the structure was in danger of collapse following the blaze, which started at about 1400 BST.
Engineers said it was unsafe to carry out repairs and signals would not be restored until at least 1900 BST. The fire has now been put out.
Eyewitnesses saw smoke drifting from the top of the mast at about 1420 BST - when pictures went off - and reported parts of the structure falling off.
Matt Carlisle, of Oxfordshire Fire and Rescue Service, said: "It has been subject to quite an intense fire at the top, so there is a possibility that the structure may be weakened."
3:44PMAll analogue (excluding Channel 5) and digital terrestrial services (Freeview) from the Oxford transmitter are currently unavailable due to a fault. Arqiva are currently working to restore services as soon as possible. We apologise for the inconvenience caused.
5:09PMTelevision broadcasts from the Oxford transmitter are currently off-air due to a fire. The fire services were deployed and we are awaiting an update from the mast operator, Arqiva, as to when services will be restored.
5:39PMTelevision broadcasts from the Oxford transmitter are currently off-air due to a fire. The fire brigade are on site and we are awaiting an update from the mast operator, Arqiva.
Arqiva, which is carrying out the work, said nobody was injured in the fire and said it would not be allowed to even look at the site to see how to rectify the problem until after 7pm, when the fire services have left.