Keep checking for insects, particularly on deciduous trees (like the oak). To overcome these unwanted elements, give each bonsai a tar wash, using a substance such as diluted Armillatox or Jeyes Fluid. You will have done this before the winter started and can now do it again. This helps eliminate the risk of infestation of pests and onset of disease.
Outdoor trees will consume little water but watering might be required in the absence of rain.
If watering is necessary, ensure that it is done early in the day; this allows time for drainage should temperatures drop below freezing at night.