A circle $\omega$ and a point $P$ not lying on it are given. Let $ABC$ be an arbitrary equilateral triangle inscribed into $\omega$ and $A', B', C'$ be the projections of $P$ to $BC, CA, AB$. Find the locus of centroids of triangles $A' B'C'$.
1. **Given**: A circle $\omega$ with center $O$ and a point $P$ not lying on $\omega$. An equilateral triangle $ABC$ is inscribed in $\omega$. Let $A', B', C'$ be the projections of $P$ onto the sides $BC, CA, AB$ respectively.

2. **Claim**: The locus of the centroids of triangles $A'B'C'$ is the midpoint of $OP$.

3. **Fixing $ABC$ and varying $P$**: Consider $P$ moving along a line perpendicular to $BC$. The projections $A'$ and $C'$ of $P$ onto $BC$ and $AB$ respectively will move with the same speed and in the same direction parallel to $BC$.

4. **Centroid of $A'B'C'$**: The centroid $G$ of triangle $A'B'C'$ is given by:
   \[
   G = \frac{A' + B' + C'}{3}
   \]

5. **Behavior of projections**: When $P$ is on $BC$, the projections $A', B', C'$ are such that $A'$ lies on $BC$, $B'$ lies on $CA$, and $C'$ lies on $AB$. The centroid $G$ of $A'B'C'$ will be influenced by the positions of these projections.

6. **Distance from perpendicular bisector**: The distance of the centroid $G$ from the perpendicular bisector of $BC$ is half the distance from $P$ to the perpendicular bisector of $BC$. This is because the projections $A'$ and $C'$ move symmetrically with respect to the perpendicular bisector of $BC$.

7. **Generalization to all sides**: By symmetry, the same argument applies to the projections onto the other sides $CA$ and $AB$. Therefore, the centroid $G$ of $A'B'C'$ will always be at the midpoint of the segment $OP$.

8. **Conclusion**: The locus of the centroids of triangles $A'B'C'$ is the midpoint of $OP$.

\[
\boxed{\text{The midpoint of } OP}
\]