All bearings that come ready fitted in a brushless motor should be of the shielded type. These will absolutely, definitely, without a shadow of a doubt, come ready-greased from the factory. Greased - not oiled. Without removing the shields "oiling" them can only mean dribbling some light oil onto the outside of the shield. At best, a tiny amount of the oil will get past the shield - at which point all it will achieve is to wash the grease out of the interior. Do that enough times and you'll end up with no grease left in there. The difference between oil and grease is that grease is designed to remain sticky/viscous at high temperatures. Oil, on the other hand, reduces considerably in viscosity as it heats - therefore oil is only effective if the part in question is sitting in a bath of it or the oil is pumped over the moving parts continuously (as happens inside a car engine, for example).
A high-speed bearing grease (like MolySlip, containing molybdenum disulphide) is what you should be using - thick, grey, gloopy stuff. The only way to get it into the bearing is to remove one shield, pack it with the grease and re-fit the shield. Which is what they did at the factory.
This is not the definitive reference on bearing maintenance, but it's the only one I can find online that is directly aimed at RC so I figured it may carry more weight in this discussion.
http://www.bocabearings.com/dropin....ss-motor-bearing-maintenance-installation.txt
Note that the procedure is to remove the bearing from its holder, remove at least one shield, remove all the existing lubricant and then re-lubricate and replace the shield. Nowhere does it say anything about applying light oil to the outside of the shield in the hope that some of it will get inside! It concludes with a rather vague statement about oil versus grease, so I'll add some further explanation:
Grease is only oil with a thickener added (called a "soap"). The purpose of the thickener is to hold the lubricant in place. At operating temperatures, the soap releases the oil so it can do its thing and then "soaks it up" again when everything cools down. The exact choice of grease is determined by the bearing "Load Factor" and "Speed Factor". The Speed Factor is calculated by multiplying the bore diameter in mm by the rpm - so a 4mm ID bearing running at 10,000rpm (typical for this application) has a Speed Factor of 40,000: in comparative terms this is a "high speed application" and suggests the use of a high-performance lubricant. The Load Factor is more complex to calculate but one factor that immediately raises it is the presence of vibration in the application, which is obviously the case here. A high Load Factor also suggests the use of high-performance lubrication. A further consideration is temperature and here, again, our motor bearings are experiencing not only their own friction-generated heat but also the heat being generated in the motor windings. Given this "extreme operating environment" and since, in this application, we can't use a recirculating oil feed or sit the bearing in an oil bath the only viable solution is to use a "high load bearing grease". We also want the bearing to be protected at start-up, before the oil has a chance to be released from the soap - so we want to use a solid additive which "micro-coats" the friction surface - Molybdenum Disulphide.
P.S. If you live/fly in a sandy area then you could replace the stock bearings with the "contact rubber shielded" type. As the name implies, they have a rubber shield that contacts both inner and outer races to seal the interior against the ingress of dirt. The penalty is slightly higher static friction but that's what you should be using if you're worried about getting grit inside the bearings.
