You Do Need power plug adapter while traveling from Guam to Oman.

Here is Why You Need An Adapter

The outlet and plug type used in Oman is G and in Guam are A and B. This is how they look like.

Guam

In Guam, outlets of type A and B are in use.

Type A

Type B

Oman

In Oman, outlets of type G are in use.

Type G