At first, I took $O$ to denote the center of the incircle inscribed into the triangle. Only a comment below clarified that you were actually taling about the center of the circumcircle circumscribed around the triangle. Therefore, I have two solutions, one for each interpretation.
Angles without a proof
I constructed your situation using Cinderella. At first I had one point chosen freely on a line, which I later moved into a position where everything fit the way it should. From the resulting figure, I obtained the following measurements:
\measuredangle ABC &= 60° \\
\measuredangle BCA &= 70° \\
\measuredangle CAB &= 50°
Warning: I accidentially swapped $M$ and $N$ as well as $B$ and $C$ in the following image. So take care to look more at the actual angles than the denoted point names.
I guess that once you know these angles at the corners, you can choose suitable coordinates and using these proove that the angles specified in your question are indeed as required.
In the above, I interpreted the angles in your question as unoriented, and measured one of the clockwise and the other counter-clockwise. I furthermore assumed $M$ and $N$ to lie between the corresponding corners of the triangle. Strictly speaking, your angles are given in an oriented fashion, and as both of them are positive, $B$ and $C$ must lie on different sides of $MN$. this leads to rather ugly triangles. Of the two possible solutions I found, the better one is the following:
The angles for this triangle are rather ugly, compared to the nice numbers resulting from the interpretation above.
Angles without a proof
This result I obtained in a similar way to the one outlined above, using Cinderella with one free point on a line adjusted till things line up as intended.
\measuredangle ABC &= 40° \\
\measuredangle BCA &= 120° \\
\measuredangle CAB &= 20°
Let us call the half corner angles like this:
\alpha &= \angle BAO = \angle OAC \\
\beta &= \angle CBO = \angle OBA \\
\gamma &= \angle ACO = \angle OCB
From $\triangle BCO$ you see that $\angle BCO = 180° - \beta - \gamma$. That is the same as $\angle MON$, so from $\triangle MNO$ you can conclude that $\beta+\gamma = 30° + 50°$. From $\triangle ABC$ you know that $2\alpha + 2\beta + 2\gamma = 180°$, so $\alpha = 90° - (\beta + \gamma) = 10°$.
Based on this angle, you can construct the triangle even without knowing the corner angles up front:
- Choose $M$ and $N$ arbitrarily
- Draw the rays $MO$ and $NO$ using the given angles to obtain $O$
- Draw lines at $90° - \alpha = 80°$ from $MO$ through $M$ and $O$ as indicated in the figure below (green). Around their intersection, draw a circle containing all points that see $M$ and $O$ under an angle of $10°$.
- Repeat the previous step for $NO$. The intersection of these two circles is $A$.
- Now $C = AM \cap NO$ and $B = AN \cap MO$.
Measuring the angles in this figure will give you the desired result up to the precision with which you performed your drawings and measurements.