I finally found out what the problem was here so I thought I would post the solution in case anyone else comes across this situation.
Firstly, I noticed this problem happening on internet explorer 11 as well while I was testing a site I have been working on. It was the only other browser displaying this ugly bold font. I switched back to Opera to debug and noticed the font family specified in the CSS was Helvetica Nueue, Helvetica, Arial, Sans Serif.
Helvetica is not a windows font by default but I remember installing a Helvetica based font which I needed for a recent project so I checked my fonts folder and found the only Helvetica font I had installed was 'Helvetica Black'.
I uninstalled the font and boom, everything back to normal, thank goodness!
It seems like if a font family is specified which is not installed on the computer New Opera and IE will use the closest font within the specified family instead of going to the next best font specified like Arial or Sans Serif (Which is what I would expect it to do).
Hope that made sense.