I wanted to delete my old oppo id account, and to do that I'll need to login into it, but I don't know the password.
Password reset requires saying when the account was created (month and year) and "tech support" can't help here either.
Is it legal to block / hide account deletion behind login in European countries? GDPR (and polish RODO) both talk about a right to data deletion, which in this case, I believe, isn't respected.
Not really since Microsoft, Yahoo (I guess), and Google dominate the email space really hard.
In terms of domains not really. Only the free-mailers use domains by one of those. The corporate users still need to set up their DNS properly for those technologies even if they use one of them as a mail hoster.
Why would OP contact OPPO using a corporate email?
It's extremely likely that they don't have their own domain since it's very uncommon for personal usage. Some absolutely do but they are in the minority.
Of course custom emails need to be set up properly, otherwise all mails would just go to spam.