Is the word oponentes the same in Brazilian and European Portuguese?

Yes

The word "oponentes" is identical in spelling, meaning, and grammatical function in both Brazilian and Continental Portuguese. The only difference lies in pronunciation. In Brazil, the vowels are generally more open and the rhythm of the word is more melodic. In Portugal, the unstressed vowels are much more "reduced" or closed, meaning they are pronounced much more softly or almost swallowed, and the overall cadence is more stress-timed.

Brazilian Portuguese Examples

1. Os oponentes do torneio são muito bons. (The tournament opponents are very good.)
2. Precisamos analisar todos os nossos oponentes. (We need to analyze all of our opponents.)
3. Os oponentes políticos não chegaram a um acordo. (The political opponents did not reach an agreement.)
4. O time enfrentou oponentes difíceis hoje. (The team faced difficult opponents today.)
5. Ela derrotou todos os seus oponentes na competição. (She defeated all her opponents in the competition.)

Portuguese (Portugal) Examples

1. Os oponentes do torneio são muito bons. (The tournament opponents are very good.)
2. Temos de analisar os nossos adversários. (We have to analyze our adversaries.)
3. Os oponentes políticos não chegaram a acordo. (The political opponents did not reach an agreement.)
4. A equipa está a enfrentar oponentes difíceis hoje. (The team is facing difficult opponents today.)
5. Ela derrotou todos os seus adversários na competição. (She defeated all her adversaries in the competition.)