Is it OK to go to postdocs for help?

In other words, can I go to a postdoc looking to talk and ask questions related to our field?

It depends on what you mean by "can", as well as the culture in your field and department.

If you are asking whether it's an ethical and reasonable thing to do, then the answer is certainly yes. You can talk with and ask questions to anyone you'd like, provided you appropriately acknowledge any assistance you receive.

If you are asking whether the postdoc has an obligation to provide substantial assistance or tutoring, then the answer is quite possibly no. It's hard to give a definitive answer, since precisely what the word "postdoc" means varies between fields and departments, but you shouldn't assume supervising graduate students is a serious part of the job unless you have some confirmation that this is true in your scenario.

If your questions are genuinely interesting and thought-provoking to the postdoc, then they will probably be happy to chat with you. You may also get a good response if you ask about topics that are truly difficult to learn about elsewhere, so that the postdoc will be sympathetic. On the other hand, if you try to lighten your workload by asking the postdoc relatively routine questions so you don't have to bother looking things up, then they will probably lose patience with you.

Yes. Helping students is commonly part of a postdoc's job. They are not obligated to help with every problem a student brings, but they probably will if they can.

Part of the idea of gathering professors, graduate students, postdocs and others is precisely to encourage exchange of ideas.