Thermal lensing is studied in monoclinic Yb:Ca 4YO(BO 3) 3 (Yb: YCOB) oxoborate crystals cut along the optical indicatrix axes. For all orientations, the thermal lens is positive. In the $Z$-cut crystal, the sensitivity factors of the thermal lens are $M_{Z-X} = 2.4$ and $M_{Z-Y} = 2.8\ \text{m}^{-1}/\text{W}$ (for $E \| X$), and the astigmatism degree is as low as $S/M = 14\%$. The positive thermal lens in Yb:YCOB is related to the large thermal expansion and strong photoelastic effect. Microchip lasers are realized with 3-mm-long $X$, $Y$, and $Z$-cut 15 at.% Yb:YCOB crystals. With a $Z$-cut crystal, maximum output power of 8.35 W is achieved at ∼1040 nm with slope efficiency of 70%. Using various crystal cuts and output coupler transmissions, multiwatt emission in the spectral range of 1033–1091 nm is demonstrated.