OBAR Bids were an invention of Bridge Pro Marty Bergen. The word OBAR is an acronym for: "Opponents Bid and Raise".
 
When playing OBAR, when opponents bid-and-raise a suit a single suit, actions in DIRECT SEAT are by partnership agreement WEAKER than standard practice.
 
For example, you hold: ♠AQ1092 ♥32 ♦J1092 ♣54, and your LHO opens 1♥. Partner passes, and RHO bid 2♥. Playing OBAR, your hand qualifies for a competitive bid of 2♠.
 
How much weaker you hand can be for an OBAR action depends on vulnerability, but advancing with a two spade bid on this hand would be a normal practice playing OBAR at matchpoints.
 
Holding ♠754 ♥KQJ1082 ♦93 ♣54, you hear LHO open 1♣ and your RHO make an "inverted minor raise (10+ HCP) to 2♣. After this inverted raise, and OBAR-style two heart bid on this would appear quite safe, and would be an important lead director.
 
Takeout Doubles are also made with very light hands when playing OBAR. Seven or eight high card points may be enough when not vulnerable, and there is room to compete at the 2-level:
 
With ♠J1093 ♥Q1064 ♦KQ82 ♣5 when next to speak after hearing 1♣-Pass-2♣, an OBAR player would be likely to double, not vulnerable.
 
In balancing seat, while standard practice normally suggests that the a competing player would overbid by about a king (and partner would do the opposite – underbidding by a king), OBAR players balance with more caution, whether the balancing action is a bid, or a double, especially vulnerable.
 
OBAR can also be played over weak opposing double raises, such as 1♣-Pass-3♣, where the opponents use inverted raises. Care should be taken, however, if the opponents are playing weak 1NT openings, since there is a real possibility that opener has a strong notrump for the minor suit opening.
 
OBAR bids are useful in that they give the OBAR pair two ways to win. And the quality of likely fit with partner is enhanced by the opposing fit.

• On part score hands, the LAW OF TOTAL TRICKS emphasizes the importance of competing when the opponents are known to have an 8-card fit.
  
• If you are outbid, the OBAR bid can provide a critical lead director.
  
• Remember OBAR only applies when opponents bid and raise a single suit. • Keep an eye on vulnerability, especially if you are advancer.
  
• Avoid OBAR with flatter hands. There’s safety in shape.
  
• When doubling, remember that the easier it will be for partner to respond at the 2-level, the safer
  
• When overcalling, remember your first duty is to direct a lead. So don’t make an OBAR bid without a good suit. If you can’t stand partner’s leading his honor from Ax or Kx, wait for some other time to bid
  
• Don’t use OBAR after a preemptive opening and preemptive raise. Have your values in direct and balancing seat.
  
• Remember: the major advantages of OBAR come from exploiting the relative safety of competing at the 2-level.