This puzzle has two parts. First, the same five letters, in the same order, will complete the partial words below. Then, what do the three newly formed words have in common?

OB _ _ _ _ _
RE _ _ _ _ _
CON _ _ _ _ _

Answers:

The letters are SERVE:

OBSERVE
RESERVE 
CONSERVE

What they have in common?

The letters SERVE can be anagrammed to form VERSE, which will also complete the original partial words:

OBVERSE
REVERSE
CONVERSE