If i'm at a location with oppressive mists and draw an encounter card, must i also make the test for the mist and potentially discard from my hands? The rules don't seem to differentiate between drawing encounter and player cards.

A similar question came up a year and a half ago about whether you could trigger Ancient Stone after drawing encounter cards.

Ultimately, we didn't get a completely conclusive answer from the devs, but there is a supposedly official ruling which is applicable regarding Dr Henry Armitage : "Unless otherwise specified 'drawing a card' always refers to drawing a card from your investigator's deck. Armitage's ability cannot be used on encounter cards."

So it's a pretty safe bet that Oppressive Mists is only supposed to refer to drawing cards from your player deck.