Bishop, Elder, Overseer, are the same office. Their purpose is to teach, instruct, help solve problems, help in managing the church.
1 Timothy 3: (KJV)
1 This is a true saying, If a man desire the office of a bishop, he desireth a good work.
2 A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach;
3 Not given to wine, no striker, not greedy of filthy lucre; but patient, not a brawler, not covetous;
4 One that ruleth well his own house, having his children in subjection with all gravity;
5 (For if a man know not how to rule his own house, how shall he take care of the church of God?)
6 Not a novice, lest being lifted up with pride he fall into the condemnation of the devil.
7 Moreover he must have a good report of them which are without; lest he fall into reproach and the snare of the devil.
Titus 1 (KJV)
5 For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain elders in every city, as I had appointed thee:
6 If any be blameless, the husband of one wife, having faithful children not accused of riot or unruly.
7 For a bishop must be blameless, as the steward of God; not selfwilled, not soon angry, not given to wine, no striker, not given to filthy lucre;
8 But a lover of hospitality, a lover of good men, sober, just, holy, temperate;
9 Holding fast the faithful word as he hath been taught, that he may be able by sound doctrine both to exhort and to convince the gainsayers.
10 For there are many unruly and vain talkers and deceivers, specially they of the circumcision:
11 Whose mouths must be stopped, who subvert whole houses, teaching things which they ought not, for filthy lucre’s sake.
12 One of themselves, even a prophet of their own, said, The Cretians are alway liars, evil beasts, slow bellies.
13 This witness is true. Wherefore rebuke them sharply, that they may be sound in the faith;
14 Not giving heed to Jewish fables, and commandments of men, that turn from the truth.
15 Unto the pure all things are pure: but unto them that are defiled and unbelieving is nothing pure; but even their mind and conscience is defiled.
16 They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate.
Acts 20:28 (KJV)
28 Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.