External oil filter tap

Rule of thumb, if the tap handle is in line with the pipe, that is the "open" position. eg petrol taps, plumbing fittings etc.

The only exception is for stop cocks (drain down taps) where it is normal for the tap to be off when in line with the pipe.  That prevents vibration from causing the weight of the tap lever to drop down and empty the tank or boiler.

Only way to be sure is to remove it from the oil line and inspect it internally