If you're thinking of "observe" as "to watch carefully," then the opposite might be:
* Ignore: To pay no attention to.
* Disregard: To pay no attention to, even if it is important.
* Overlook: To fail to notice.
If you're thinking of "observe" as "to comply with a rule or law," then the opposite might be:
* Violate: To break or disobey a rule or law.
* Disobey: To refuse to obey.
* Defy: To openly resist or oppose.
If you're thinking of "observe" as "to make a remark," then the opposite might be:
* Remain silent: To not say anything.
* Keep quiet: To not speak.
* Stay mum: To keep silent.
The best opposite word for "observe" depends on the specific situation and the meaning you want to convey.