It's one thing to just chalk off things to "expert opinion" and another to actually look at the study in question, for some quantifiable effect, even if just in probability. I wasn't able to locate this last (Oxford) study yet, but it's far from being the first in this line of research. I found a much older study with ~1000 citations in Google Scholar:
The summer of 2003 was probably the hottest in Europe since at latest AD 1500, and unusually large numbers of heat-related deaths were reported in France, Germany and Italy. It is an ill-posed question whether the 2003 heatwave was caused, in a simple deterministic sense, by a modification of the external influences on climate—for example, increasing concentrations of greenhouse gases in the atmosphere—because almost any such weather event might have occurred by chance in an unmodified climate. However, it is possible to estimate by how much human activities may have increased the risk of the occurrence of such a heatwave. Here we use this conceptual framework to estimate the contribution of human-induced increases in atmospheric concentrations of greenhouse gases and other pollutants to the risk of the occurrence of unusually high mean summer temperatures throughout a large region of continental Europe. Using a threshold for mean summer temperature that was exceeded in 2003, but in no other year since the start of the instrumental record in 1851, we estimate it is very likely (confidence level >90%) that human influence has at least doubled the risk of a heatwave exceeding this threshold magnitude.
And more recent research for the US:
The graph shows the fraction of heat waves for which models projected global warming would be the primary contributor. https://i.sstatic.net/g1MzP.png
(Cannot the include the image presently because of a Stack Exchange error. Click the link for now.)