In this study, we investigated orbital period variations of two contact binaries V343 Ori and FZ Ori based on published minima times. Using the O-C analysis, it was found that both systems indicate orbital period increase. Mass transfer from less massive component to more massive component was used to explain increases in the orbital periods. On the other hand, the secular changes in their periods can be a sign of the thermal relaxation oscillation. In the O-C diagram of FZ Ori, periodic variations also exist. Cyclic periodic changes can be explained as being the result of a lighttravel time effect via a tertiary body around the eclipsing pair. The minimum mass of probable tertiary component around FZ Ori was found to be 0.63 M⊙. In addition, the cyclic variation may be evidence of magnetic activity of the components, which are late-type stars. [ABSTRACT FROM AUTHOR]