ὁ/ἡ βοῦς the ox/the cow

 
SINGULAR
  
 nominative             ὁ    βοῦς  μέγας ἐστίν.
 genitive  οὐχ ὁρῶ τὸν    τοῦ    βοός     δεσπότην
 dative  προσχωρῶ   τῷ    βοΐ   
 accusative  ἐλαύνει     τὸν    βοῦν    εἰς τὸν ἀγρόν.
 vocative   ὦ      βοῦ     μὴ μένε ἀλλὰ πόνει!
  
PLURAL
  
 nominative          οἱ    βόες     ἰσχυροί εἰσιν.
 genitive τὰς βοὰς τὰς     τῶν    βοῶν    
 dative  προσχωροῦμεν   τοῖς    βουσίν     
 accusative   οὐ φοβοῦμαι     τοὺς    βοῦς    
 vocative   ὦ      βόες    ἕλκετε τὰ ἄροτρα, ταχέως.
  
 
These forms look worse than they actually are:
 
nom sg, voc sg, acc sg and pl, as well as dat pl all have ου 
The other forms are regular, except that in writing dative singular βοΐ has to take a diaeresis, otherwise it would be read as "boy".
  •  ὁ/ἡ βοῦς
  •  ὦ βοῦ 
  •  τὸν/τὴν βοῦν 
  •  τοὺς/τὰς boῦς 
  •  τοῖς/ταῖς βουσί(ν)