The objective of this study was to assess in-vitro interfacial cavity adaptation of different adhesive/composites systems in class-II restoration using SS-OCT and to confirm the findings with a confocal laser scanning microscope (CLSM). Standardized rectangular-shaped class-II cavities were prepared on 24 extracted human molar teeth, bonded with adhesive and lined with flowable composite. Cavities were further restored by bulk filling using packable composite. Specimens were divided into four groups (n?=?7) according to restorative materials used; PrimeFil with Estelite Posterior composites (PF; Tokuyama Dental), Clearfil Tri-S Bond Plus with Majesty LV flowable and Majesty Posterior composites (TS; Kuraray Noritake Dental), Adper Easy bond with Filtek Z350 XT flowable and Filtek Z350 XT packable composites (AE; 3?M ESPE) or Palfique Bond adhesive with Estelite Flow Quick flowable and Estelite Posterior composites (BF; Tokuyama Dental). To evaluate the cavity adaptation of these restorative materials, tomographic images of pulpal (P) and gingival (G) walls were obtained by SS-OCT with a central wavelength of 1330?nm, followed by cross-sectioning and CLSM observation to confirm OCT findings. The results showed a significant increase in the backscattered reflection was attributed to the interfacial gaps. AE groups demonstrated the highest interfacial adaptation percentage (p??0.05). BF group presented the least adaptation percentage, and the gaps were frequently observed at the adhesive/composite interface. In addition to the sensitivity of the SS-OCT to the interfacial gaps, the success of flowable lining relies on the interactions between the adhesive/tooth and adhesive/composite substrates.