My novel is written in the third person. When changing POV between different scenes (or chapters), is it OK if the parts for different POV characters use different tenses: some - the past tense, and others - the present tense?
It's a bit dicey. You'd have to have a good reason for it.
- If Preston's POV is in the present, but Paola's POV is in the past, is the story happening in Preston's time? Is Paola relating things in flashback?
- Is Preston an alien or someone who experiences time in a non-linear fashion, as compared to Paola who is a more bog-standard human?
- Are you experimenting with storytelling techniques?
Shifting POV is fine; it's done all the time. Shifting tense usually indicates a break from the narrative. Dreams might be in present tense, for example, or a flashback from a present-tense story would be in past tense. But I think you would need a really compelling reason for Preston and Paola to have two different tenses in an otherwise straightforward narrative.
As Lauren said, it can be dicey. But it can definitely work! I wouldn't recommend doing the switches within the same chapter, because that would get confusing very quickly and hard to follow. What I've seen that seems to work is a POV and tense switch from chapter to chapter or larger sections in general. The main obstacle is that the POVs need to be consistently linked somehow. This can be done by using a setting, such as a specific house inhabited by one character in the past and a different character in the present, or an object, such as an antique found by one character that was owned by another when it was new.