There is no such thing as a “perfect crystal” in nature, every material is known to have defects. Depending on the kinds of defects, we can gain advantage by defect engineering to improve or manipulate a material’s properties. On the other hand, defects can be deleterious for applications, especially in the semiconductor field. Studying novel oxides therefore requires understanding the defects which can be done with scanning transmission electron microscopy (STEM), capable of giving atomic-scale information. In this thesis, I observed defects in novel oxides: Ga2O3 and metallic Delafossites, with the aid of STEM. I probed the co-existence of multiple metastable phases of Ga2O3 while for Delafossite materials I have confirmed the intrinsic defects in highly conductive bulk PdCO2 and PtCoO2 that explains the reason behind the drop in the conductivity of PdCoO2 thin films. In the first part of my thesis, I will mainly discuss about the Ga2O3 family that is gaining increasing interest as the next generation ultra-wide band gap semiconductors. This requires high crystallinity, pure phase thin films for controllable electronic properties. However, we observe inclusions of _-phase that has the highest formation energy among all polymorphs, appearing in the most stable _- and _-phase thin films grown by molecular beam epitaxy (MBE). Such co-existence of phases is problematic, especially when the most metastable phase is formed within the film. The second part of my thesis focuses on metallic Delafossites, which are known to be the most conductive oxide to date. They were first synthesized back in 1971, however due to difficulty in growth and the resulting small crystal sizes, it has gained back interest only recently. Here I will mainly talk about PdCO2 and PtCoO2 that have resistivities as low as 7.5 nΩ·cm, corresponding to a mean free path of 20 _m at low temperatures. From STEM measurements, we find the defect density to be exceptionally low (as low as 0.001%), supporting theories that the low scattering is a result of high crystal perfection, rather than some special new mechanism suppressing scattering. Here I present the first observation of intrinsic defects in bulk PdCO2. For applications, thin films of PdCO2 has been grown recently by MBE which shows increased resistivities as it now confronts boundary conditions and strain effects. With the aid of STEM, I discuss how various defects can affect the electronic properties.