Here is the relevant card text, for discussion:
Guide: "The heroes may choose to roll one less or one more power die each time they check for an encounter."
Big Trouble: "Place this card in the graveyard before checking to see if the heroes have an encounter. The check automatically indicates an encounter." [additional text omitted as it is irrelevant to this question.]
In my opinion, it seems clear that Big Trouble takes effect first. Big Trouble is used "before checking to see if the heroes have an encounter" while the guide is used "each time [the heroes] check for an encounter." Even if you want to consider them as being used at the same time, nothing about the text on Big Trouble suggests to me that it requires the heroes to roll at least one die. It doesn't say "take one of the dice rolled and turn it to a facing that indicates an encounter." So yeah, the OL can still use Big Trouble in this case. In fact, he should really be declaring his intent to use Big Trouble before the heroes decide whether to use the guide or not.