Newbie ammo question

Yes there is a "break in" period to most/all air guns. 

As far as a "best" pellet, you should shoot different weights/brands and see what you particular rifle likes.

cally is right 

​all riffles unless pre owned have a break in where the actions need to become a bit smother as well as the barrel needs to be shot probly on average about 100 shots before the groups tighten up a little
as for pellet again he's right every barrel can be picky and finding what weight and brand works best s key
​for a .177 a big thing for you is going to be fps so a heavier pellet might work well for you unless you get lucky but this is also depending on how far your going to shoot
​if your going to shoot only 30 yards a lot of pellets will work well but pushing out past 50y you'll be testing a good bit

Cally and Gorgata are both right on, Here are a couple other things: steer clear of the big box non-lead pellets (PBA) they usually give poor accuracy in a springer like yours. Domed pellets usually shoot best in most rifles at longer distance, for close range (10 yards) target shooting a flat head wadcutter may be best. Probably the best pellets available at your local store will be Crosman Premier hollowpoints in the screw tin, if your gun likes those, you're good to go! Some of the best brands to try are JSB, H&N, and RWS.
If your rifle has open sights it would be a good idea to test pellets for accuracy using just the open sights. Once you've got the rifle grouping THEN mount your scope and sight in, the recoil of a springer can move the scope and/or mount and that will make you crazy! (Ask me how I know this, LOL)
A springer like the Octane shouldn't be held tight like a firearm, research the "artillery hold" and experiment to see what hold your rifle prefers. (hope you have lots of ammo!)
Finally if your groups open up suddenly, check that the stock screws are snug.
Shoot it a lot and have fun!
John

Also you probably already know.... "Dry" firing is a big no no with spring guns.

remember do not get discouraged while testing if a particular pellet is not working well. Part of the air gunfun is tinkering, have fun with your testing!

snosurfer I see the Umarex Octane is a powerful gas ram rifle. With that is mind heavier pellets will probably be better than lighter pellets. Also depending on how far you shoot the dome pellets should do better accuracy wise once you get out past 20 to 25 yards. At Straight Shooters they did a review of your rifle which may give some insight and tips for you. Bill
http://www.straightshooters.com/umarex-octane-.177-combo.html