Examples
Example 1
Oh, I thought everybody was going to the party dressed as their favorite Hunger Games character. Now I just feel like a dork carrying around this bow and arrow.
In this example, the interjection oh is used to express surprise. Sure, the speaker feels silly now, but if the party is attacked by muttations, those arrows will come in very handy.
Example 2
You can eat seven saltines in one minute? No way!
Consuming seven saltines in sixty seconds is virtually impossible unless you're Peyton Manning. On the other hand, it's totally possible to identify no way as an interjection in this example. Here's it's being used to express the speaker's disbelief.
Example 3
Holy guacamole! Susan's new haircut makes her look like Justin Bieber circa 2009.
Here, the phrase holy guacamole is an interjection used to express surprise, or perhaps shock, at the fact that anyone would pay a stylist to make them look like one of Canada's most infamous musical exports.