The following can be grounds for opposition:

The mark does not meet the definition of a trademark proprietary rights
The mark is generic
The mark is not distinctive
The mark is descriptive
The mark is against public policy or principles of morality
The mark consists of a geographical indication
The mark is misleading, deceptive or disparaging rights in a personal name
The rights under Article 6ter of the Paris Convention
The rights under Article 8 of the Paris Convention
The rights under Article 6septies of the Paris Convention
The rights under Article 6bis of the Paris Convention