A 'scope is used to see waveforms. Waveforms must be displayed linearly, not in dB's.
db's are ratios of steady levels that can be seen on an audio or RF multimeter.
I think this ^ is very good point (about linear vs logarithmic measurements) in a great thread about an important topic (dBs).
Here is another attempt at helping the OP - others should feel to correct whatever I get wrong.
dBs are useful in making measurements that have relatively large dynamic range, ie lots of difference between the largest and smallest values being measured or considered. dBs become most valuable when the dynamic range being examined needs to be expressed in orders of magnitude (factors of 10, ie 10x, 100x, 1000x, etc). To make dBs conveniently useful, as stated in other posts in this thread, dBs are often/generally (but not always explicitly) referenced to some specific value such 0dBm = 1mW and therefore we can quickly recognize that 30dBm = 1 watt. (The point made above about impedance assumptions in the use of dB for power is a great example of some assumptions being present but not always explicit, which can lead to confusion when first getting a grip on the various uses of dB; just something to be aware of.)
Back to oscilloscopes. When using an oscilloscope we might be looking at somewhat small values such as 1uV or somewhat large values such a 100V but we typically don’t use a scope for a task such as looking see if something is 100.000001V, or if one signal is a 100V vs another at 1uV. In fact we generally use probes that are specified within an order of magnitude such as 10x (or sometimes 1x or 100x).
So while an oscilloscope is a fantastically useful piece of test equipment (occasionally even referred to as the king of test equipment) it is not one of the types of test equipment most likely to be used for measurements of values frequently designated with the dB nomenclature.
To be sure an oscilloscope might be used to measure in ranges from, for example, seconds to picoseconds on the (horizontal) time axis, but in terms of the (vertical) amplitude scale dBs don’t get used much (hardly at all?).
In contrast, in the case of spectrum analyzers we can look also at the horizontal scale in terms of sweeps of full seconds to small fractions of a seconds as we look at relatively larger or smaller swaths of bandwidth, and we might greatly reduce the span of frequency to see with more detailed resolution. So again, time might range from seconds to billionths or maybe trillionths of seconds. However on a spectrum analyzer we might look at 10 orders of magnitude in amplitude (or more depending on how the reference graticule is set). So on a spectrum analyzer dBs get used regularly on the vertical scale. With a spectrum analyzer we might be measuring and comparing amplitudes in the range of +30dB to -160dB.
In summary, a key concept to help make dBs come into better focus is “dynamic range”, ie the need to not only measure but also readily compare values (of whatever is being measured) across multiple orders of magnitude.
The more you get your head around dBs the more profoundly you can get your head around the enormous potential for dynamic range and scale for various realms in various systems. Kinda metaphorically (so similar to but not exactly) like trying to quantify and compare how big is an electron relative to an atom or an atom compared to a solar system or a solar system compared to the Universe.