Consider a point $O$ in the plane. Find all sets $S$ of at least two points in the plane such that if $A\in S$ ad $A\neq O$, then the circle with diameter $OA$ is in $S$.

[i]Nikolai Nikolov, Slavomir Dinev[/i]
1. **Lemma:** If \( A \in S \) and \( O \neq A \), then a closed disc with radius \( OA \) belongs to \( S \).

   **Proof:** By the conditions of the problem, we have that the circle \( w \) with diameter \( OA \) lies in \( S \). Take any point \( B \) in the interior of this circle and draw a line \( l \) perpendicular to \( OB \) at \( B \). Take \( C \in l \cap w \) (note that the latter is non-empty) and draw the circle with diameter \( OC \) (since \( C \in S \)). Note that \( B \) lies on this circle. Hence \( B \in S \). Q.E.D.

2. **Case 1:** There exists a ray \( z \) passing through \( O \) such that \( \forall x \in \mathbb{R}^+ \ \exists X \in S \colon X \in z, \ OX > x \).

   - Draw a line \( y \perp z \) such that \( y \cap z = O \) and denote the half-plane whose boundary is \( y \) and in which \( z \) lies by \( U \).
   - Note that every point that lies in \( U \) but does not lie on \( y \) is in \( S \) (using the choice of \( z \), we prove it in the same fashion as we have proven the lemma).
   - Now note that every point that lies on \( y \) is in \( S \) (take a point in \( U \) sufficiently close to \( y \) and sufficiently far from \( O \), draw a circle with diameter whose one endpoint is the chosen point and the other is \( O \), and use the lemma).
   - In the same manner (taking \( y \) instead of \( z \)), we prove that every point in each of the two half-planes in which \( z \) and \( -z \) divide the plane belongs to \( S \).
   - Finally, we prove that \( -z \) lies in \( S \) (again analogously). Hence \( S \) coincides with the plane.

3. **Case 2:** There does not exist such a ray \( z \) as aforementioned, and consequently \( \exists m \in \mathbb{R}^+ \colon \forall A \in S \ \ OA \le m \).

   - Let \( m \) be as minimal as it can be. In other words, \( m = \sup \{ |OA| \colon A \in S \} \).
   - Perform inversion centered at \( O \) with radius \( m \). Denote \( X' \) as the inversive image of \( X \) and \( S' \) as the inversive image of \( S \).
   - Note that the conditions of the problem imply that for every \( A' \in S' \), the line \( l' \perp OA' \colon A' \in l' \) lies in \( S' \), and our lemma implies that all half-planes which have \( l' \) as the boundary and do not contain \( O \) lie in \( S' \).
   - Choose \( l' \) sufficiently close to an open disc centered at \( O \) with radius \( m \). Fix \( \epsilon > 0 \) and set \( l_0 = l' \). By \( A_i \), we will denote a point on \( l_i \) such that \( OA_i \perp l_i \).
   - Choose \( B_0 \in l_0 \colon B_0A_0 = \epsilon \) and let \( l_1 \) pass through \( B_0 \) perpendicularly to \( OB_0 \). \( l_{n+1} \) is obtained from \( l_n \) in the same fashion. We just have to choose such \( B_n \) that \( \angle A_{n-1}B_nA_{n+1} \) is obtuse.
   - Note that in such a way, we step by step rotate around the circle of inversion by a fixed angle. Note that every \( l_i \) and a half-plane which has the boundary \( l_i \) and does not contain \( O \) lies in \( S' \) (we get this using the lemma and the aforementioned equivalence of the conditions of the problem).
   - In such a way, we rotate around the circle and get that all planes without the open disc centered at \( O \) with radius \( m \) lie in \( S' \). Hence, an open disc centered at \( O \) with radius \( m \) lies in \( S \). By the definition of \( m \), no other points different from those lying on the boundary of the closed disc centered at \( O \) with radius \( m \) can lie in \( S \).
   - Let \( T \) be any subset of this boundary and note that we can have \( T \subset S \) and \( S \) will satisfy the conditions of the problem.

4. **Conclusion:** \( S \) is either the entire plane or a union \( T \cup U \) where \( U \) is any disc centered at \( O \) without its boundary and \( T \) is any subset of points of this boundary. Clearly, all such \( S \) satisfy the conditions of the problem.

The final answer is \( \boxed{ S } \) is either the entire plane or a union \( T \cup U \) where \( U \) is any disc centered at \( O \) without its boundary and \( T \) is any subset of points of this boundary.