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I'm forming an antenna from insulated copper wire.
Insulation property is to block electric field, if I am not mistaken.
Should I strip the insulation from the wire to allow better receiving of radio waves? It is for DBV T2 range.
I've heard that oxidation of copper surface might reduce strength of reception.
The insulation won't impede the electric field in any significant way. It blocks the motion of charged carriers, but the electric and magnetic fields can exist without that mechanism.
Stripping the insulation will, as you suspect, result in oxidation of the surface of the copper. While this patina does repel water, preventing further corrosion, at RF the current in a wire tends to travel along the surface instead of the interior. If the surface has a higher resistance, this will impede your reception (and/or reduce your TX power).
The best bet is to leave the insulation on, although if exposed to the elements, you'll want to be sure your insulation is rated for that. Ultraviolet light and weathering can make most plastics brittle.
I found an article Insulated Wire and Antennas, Rudy Severns N6LF, January 2017. It tells there is influence on reception (like The insulation makes the wire electrically a little longer (≈1.5%) it is about Ri) but the conclusion is:
Looking at all these considerations it's clear that in general leaving the insulation on the wire is pretty benign and loss due to the insulation, either new or old, does not seem to be significant. However, it was shown that in certain cases, mostly related to GP- verticals with sparse radial systems there can be a substantial impact. However, that really occurs only when very few radials are used. These problems tend to go away as the radial count is increased to twelve or more for elevated radials and 16-20 for ground surface or buried radials.
Besides it influences mechanics characteristics (like weight):
Mechanical issues Leaving the insulation on the wire increases the weight of the wire and, if there is icing, the increased diameter could lead to even more weight. From a corrosion point of view insulated radials are very likely to last longer than bare radials, especially for ground surface or buried radials
The reason you want to leave wire insulated could be banal:
Stripping a few hundred feet isn't a serious chore but if you're laying out a 160m radial field with thousands of feet of wire then stripping would be a chore.
