In league, you usually bowl on the standard oil pattern ("shot") of that bowling center ("house"). Generally, a house shot has most of its oil volume concentrated in the middle of the lane, between the second arrow in from both gutters, and running from the foul line to about 2/3 of the way down the lane. This is why bowling balls thrown over the second arrow will be more likely to strike - they are "guided" by the edge of this oil concentration right to the pocket. (Of course, your results may vary!) Skilled bowlers will generally score very highly on a house shot.
While it may seem like the house shot in one bowling center is quite different from the house shot in another bowling center, the difference is actually quite minor compared to the differences between any house shot and the more challenging oil patterns.
A more challenging oil pattern has the oil applied more evenly across the lane, at a distance that could be more OR less than 2/3 of the way down the lane, using a total oil volume that is generally more BUT could also be less than a house shot, and with individual parts of the oiled area having more oil than a house shot AND some other parts having less.
Hitting the pocket on a more challenging pattern will be much harder. Your chance of getting a strike will depend much more on which bowling ball you use (solid, hybrid, or pearl cover; symmetrical or asymmetrical; etc.), where your slide foot ended up at the foul line, what your target was, the point you were aiming for the ball to start turning ("breakpoint"), what position your hand was in when the ball rolled off your hand, the speed with which you threw the ball, and how many times the ball revolved as it rolled down the lane (your "rev rate").
Complicating matters is that, with players playing different lines to the pocket, and with everyone's bowling balls picking up oil as they go down the lanes (especially the balls with non-shiny covers) and/or pushing it down the lane (especially plastic spare balls and urethane balls), the location and consistency of the oil will change with each game and perhaps even with each frame. You will need to know how to adjust for these changes on the fly, keeping from being discouraged if you can't find an adjustment that works even after a few frames.
Success will be next to impossible without practice and experience on challenging patterns. If the tournament has disclosed in advance which oil pattern will be used, your bowling center may be willing to put down that pattern on one or more lanes at a specific time so that you and your fellow competitors can practice on it.
But, some tournaments, such as the annual USBC nationals, no longer disclose the patterns in advance. The best way to prepare for nationals, and for any challenging oil pattern for that matter, is to join a "PBA Experience" or "sport" league. This type of league gives you the chance to bowl on a variety of challenging oil patterns, thereby building your confidence in being able to adjust to any oil condition you will face. But be forewarned: Your average in this type of league will likely be 25 or more pins lower than your usual average because the conditions are that much more difficult.
Overwhelmed at this point? Click on the photo below to watch a fun 8-minute video from Vox Magazine that will visually show you the basics covered above.
Once you are comfortable, your next step will be to learn and understand how to read oil pattern diagrams so that you can make the necessary adjustments to keep striking. For help with this, see our page on How to Read an Oil Pattern Sheet.