Om is used to describe how long until something happens OR something that is repeated several times.
In your example: “Det er viktig for helsa om høsten og vinteren” - It is something that is repeated several times - Every fall/autumn and winter it is important for health.
I is used when talking/writing about the current or the nearest season or time of day, and can be present, future or past, also when talking about months and years, and how long something lasts or has lasted.
In your example: “Ellers har det vært mye dårlig vær i høst” - We’re talking about the current or the nearest season which is present or past.
We have four seasons:
- Winter (Vinter)
- Spring (Vår)
- Summer (Sommer)
- Autumn/Fall (Høst)
We are talking about the nearest:
- Something that just was (Past)
- Something that is now (Present)
- Something that is upcoming (Future)
We are talking about something in general as mentioned earlier:
- Every winter, spring, summer, fall/autumn (Hver vinter, vår, sommer, høst)
Note the ending en in “vinteren”, “våren”, “sommeren”, “høsten” when we use the preposition Om. When using the preposition I, we don’t have any ending; “vinter”, “vår”, “sommer”, “høst”.
Another preposition we use is Til which literally means To.
Til is used to describe a future period of time, during which something is going to happen.
Here, we also use the ending en in vinteren etc.
“Til sommeren kommer Coco på kino” - “Coco is coming to the cinema/movies next summer”
We’re talking about something that is upcoming.
I hope this answered your questions and concerns. It was difficult to explain why certain stuff is how it is; Norwegian is a complicated language grammar-wise