Even if a woman doesn't choose to have a hijaab, wants to drink, eat pork and wear a bikini, she can still be a Muslim in the eyes of God if she believes in a certain phrase (the 1st kalimah), but she may not be a good example of a Muslim, no.
It may be sen as oppressive just because it may seem that these women are 'forced' to wear these by family, husbands, etc (but usually, it is of the woman's own free will, for only the pleasure of God). Also, some may think that the hijaab, as it is covering up a woman's 'attractive' feature, then it may be restricting her to live and appear how she would like to - but again, it is most often her own choice to adopt the covering.