I think this is pretty much entirely a DM call. First of all, though, note that the meaning of 'combustible' can vary somewhat, depending on to what extent you think it means 'easy to ignite' as opposed to just 'burnable'. That said, I just try to think in very rough terms about how hot I think the flame is, how long it is in contact with the item/substance in question, and how easy, more or less, the item is to ignite (which comes down to how hot it has to be and its heat capacity).
On one end of the spectrum, the flame from produce flame "harms neither you nor your equipment" and continual flame "produces no heat". I kinda don't think those are going to ignite anything.
Highly damaging but brief fires, such as flame strike or fireball, will certainly char paper or very thin wood, and might set it ablaze; however, because they are so brief, thicker wood or cloth would be lightly charred or singed. For oil, it depends on what you think 'oil' is in your world: if you think it is like kerosene, then it probably ignites; if you think it is more like a heavy fuel oil, whale oil, or vegetable oil, then (IMO) probably not.
Something like flame blade depends on how it is used. If it just strikes something, it might leave some soot, but probably won't set it on fire. But if you want to hold it against a piece of paper, kindling, or a torch - sure, it will set them on fire. I'd even let it work on something harder like a piece of coal, if you were patient.
Several spells specifically refer to their effect on 'flammable' objects. Personally, I don't think this adds much since it just moves the question to what materials you consider 'flammable'.