It might help [and it might not!] to think of the 'correct' exposure using the concept of 'zones', or narrow ranges of densities or 'grays.' How wide are these zones? Well, if you double the shutter speed [eg, from 1/250 to 1/500] or reduce the aperture by one full f stop [f8 to f11], you have moved the exposure one full zone. If you cut the shutter speed in half [eg, 1/250 to 1/125] or increase the aperture by one full f stop [f8 to f5.6], you've moved one full zone in the opposite direction.
Read that paragraph again to be sure you've got it before continuing.
Next, think of b&w film. [Yes, I know you shoot color film, but thinking in b&w makes the following a bit easier to understand.] B&w film records shades of gray. Now think of it as recording zones of grays. In general, a properly exposed and developed b&w film can record a total of 9 zones of grays. Let's call them zones 1 through 9. The zone at one end is almost black. The zone at the other end is almost white. The zones from 1 to 9 form a 'scale' of grays.*
OK, here comes a change-up pitch! Right smack center in the middle zone, Zone 5, you'll find the same shade of gray as that on an 18% gray card.** So whenever you read about a meter trying to record something as 18% gray, the meter is simply trying to set an exposure safely in the exact middle of the range that the film can record. It's also why you can use an 18% gray card to determine an exposure for a scene containing a whole range of grays [zones].
Now let's wrap this puppy up and go home.
The deep grays at one end of the scale are those of the shadows. The light grays at the other end are those of the highlights.
In a scene where the range of grays, from the deepest shadows to the brightest highlights, does not exceed 9 zones [or 9 f stops, or 9 shutter speeds -- it's the same thing***], you set your exposure for zone 5 -- 18% gray. Your film will capture the full range of grays.
On an overcast day, when there's less contrast, the full range of grays in a scene may be less than 9 zones. Let's say that it's only 7 zones. The same procedure applies here also. The safe spot is 18% gray -- zone 5. You've got a safety zone [Sorry!] at each end of the scale.
However, and this is one big however, if the range of grays exceeds 9 zones [bright sunlight on snow with deep shadows under fir trees], you have to make a choice on which zones you choose to lose. You can reduce the exposure [faster shutter or smaller aperture] and have a better chance of capturing detail in the snow [Deepest shadows will go completely black - no detail,] or you can increase the exposure [slow the shutter down or open up the aperture] and have a better chance of capturing detail in the shadows [Snow will go blank white - no detail.]
When you've got the time to do so, you can meter the shadows, meter the highlights, and then decide on your exposure. [This is where a hand-held meter comes in -- especially if you've got your camera set up on a tripod!] If the range is 9 zones or less, go for the middle zone. If the range is greater than 9 zones, make your choice on what you are willing to lose. Not sure? Take multiple exposures, each separated by a zone [This is called 'bracketing' an exposure.]
If you don't know the reading for the deepest shadow and brightest highlight, you go with a 'best guess.' It's important then to know how your camera's exposure meter 'sees' the scene. Past experience with similar situations is also important. Chances are, and I haven't checked this, the OM2 tends to read the center of the scene.
It's time now to make a visual adjustment to your thinking. The real world is in color, not b&w. [Yup, I knew that all along!] You will have to look at a scene in terms of the brightness or darkness of the colors to determine the 'shadows' and 'highlights.' The deepest, most saturated colors in the lowest light are shadows. The least saturated colors [nearest to white] in the brightest light are the highlights. Color films differ from b&w in the number of zones they can capture. I'll leave it to you to find out about your particular film. This doesn't change a thing. You still go for the center zone unless the range of brightness in a scene exceeds the film's capabilities.
Believe it or not, with time this sort of thinking becomes an almost automatic process.
*You'll note I haven't said which end is which! That's because the zone which is lightest on a negative forms the darkest image on the print. You've got enough on your plate right now without trying to keep that all neat and tidy in your mind!
**Thinking of 18% gray as the middle is no stranger than thinking of middle C on a keyboard. The center of the scale is D, not C. In school, C is considered as a middle grade, but C is only the third letter in a 26 letter alphabet.
*** The ISO film rating system is also in zones, if you wish to think of it that way. Go from an ISO50 to an ISO100 film and you can capture one more zone toward the dark end at the same exposure. ISO numbers double for each increase in zone - 50,100,200,400,800, etc. From ISO50 to ISO400 is a shift of 3 'zones.'