The former foreign minister told Spanish radio station Ondacero: “I was in the European Commission when many countries asked to join. I think it was hurried.
“Countries like Finland, Sweden and Austria were natural. But then Germany fought really badly to get Slovenia in.
“The German Vice-President said that it would not create problems.
“Maybe not that country, but then there were two islands in the Mediterranean, and then countries that fought against the Soviet Union like Estonia, Latvia and Lithuania.
“I think it was a rushed decision. They did not meet all the requirements. I think it was one of the mistakes.”
He said that long-term impact of this rushed decision was showcased in the French election, which was evidence that “we may have given up too much”.
The former Spanish official added: “It must be clearly stated what must remain within the European institutions and what should be the responsibility of each state.”
The Spaniard also urged his country’s leaders to exploit the gap in Europe left behind by Brexit.
Mr Oreja said Spain could now become a major player in the EU, alongside Germany, France and Italy.
The former politician said that Spain did not suffer from the same level of eurosceptism that Britain does.
He said that the country had recieved a lot, both financially and institutionally, and would always be grateful to Brussels.
In the interview, Mr Oreja said that Spain will have recieved around £168 billion from the EU by 2019.
He said: “Now we can do better. There is a hard core in which Spain can get in with the departure of Great Britain.
“I believe that in that group of 4 countries, one of them must be Spain and must play a strong and decisive role.
“We have much to say in the European institutions.”