The 27-year-old left-back has been linked with a possible departure from the Greek champions since last summer, although Olympiacos ultimately decided against sanctioning his transfer at the time.
The situation could now change, with the Greek club reportedly prepared to listen to offers for the Nigeria international.
According to transfer expert Ekrem Konur, Olympiacos are willing to sell Onyemeachi but are expected to demand a significantly higher fee than the amount they paid to acquire him from Portuguese side Boavista.
Olympiacos signed the defender in January 2025 for €2 million, and the club are now reportedly looking for around €6 million for his services.
Real Betis' interest comes as Onyemeachi continues to draw attention following his performances in Greece. The Spanish club are understood to be monitoring his situation, although there is currently no concrete offer on the table and no active negotiations between the two clubs.
That means any potential move remains at an early stage, with Olympiacos yet to receive a formal proposal for the player.
Onyemeachi's current situation is not his first experience of attracting interest from Spain. During his time at Boavista, the defender was previously linked with moves to La Liga, with Las Palmas and Getafe among the clubs reported to have shown interest in him.
His potential availability could therefore revive interest from Spanish clubs, particularly if Olympiacos follow through with their reported decision to consider offers for the left-back.
For now, Real Betis' interest remains exploratory rather than concrete, while Olympiacos appear to have placed a €6 million valuation on a player they acquired for a considerably lower fee less than two years ago.