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

Yes

The meaning, spelling, and grammatical function of "ontem" are identical in both Brazilian and European Portuguese. The only difference is pronunciation. In Brazilian Portuguese, vowels tend to be more open and clearly articulated. In Continental (European) Portuguese, unstressed vowels are often reduced or "swallowed," meaning the "e" in "ontem" is pronounced much more softly or almost disappears, making the word sound more "clipped" than its Brazilian counterpart.

Brazilian Portuguese Examples

1. Eu vi ele ontem na rua. (I saw him yesterday on the street.)
2. Ontem foi um dia muito cansativo. (Yesterday was a very tiring day.)
3. Você me ligou ontem à noite? (Did you call me last night?)
4. Nós comemos pizza ontem. (We ate pizza yesterday.)
5. Ontem eu esqueci meu guarda-chuva. (Yesterday I forgot my umbrella.)

European Portuguese Examples

1. Eu vi-o ontem na rua. (I saw him yesterday on the street.)
2. Ontem foi um dia muito cansativo. (Yesterday was a very tiring day.)
3. Ligaste-me ontem à noite? (Did you call me last night?)
4. Nós comemos pizza ontem. (We ate pizza yesterday.)
5. Ontem esqueci-me do meu guarda-chuva. (Yesterday I forgot my umbrella.)