Well, now I knew about fending, defending and fencing. What about offending? Offend comes from ob- + -fendere (found only in compounds; < the same Indo-European base as Hittite kuenzi ‘he strikes, kills,’ Sanskrit han- ‘to strike, kill, put an end to.’ Hmm. The OED’s editors are much more chatty about –fendere in this entry than they were when discussing the etymology of defend.
The prefix ob- is from classical Latin ob (also op in inscriptions) ‘in the direction of, towards, against,’ among other meanings. Ob-? It’s of- in this case. That’s because “the b of classical Latin ob is assimilated to certain consonants, becoming oc- before c- , of- before f- , op- before p- , and apparently o- before m- (in omittere ).”
In the 14th century offend meant ‘to strike with the feet against something, to stumble.’ Just as early it was used figuratively: ‘To make a false step or stumble morally; to commit a sin, to fail in duty; to do wrong, transgress, infringe a rule; (Law) to commit a crime, break the law…to wrong (a person). In the first source cited by the OED, the Wycliffite Bible (1382), to be offended meant ‘to be displeased, vexed, or annoyed.’ Chaucer used offend to mean ‘to hurt (someone’s) feelings’: hoping to never “thynke or seye [anything] That yow myghte offende in any tyme.” He also used it in the sense of assailing or assaulting (figuratively though others had used it literally): “I am‥with loue offended moost That euere was any lyues creature.”
Offense in the sense of ‘an attacking team or player, the attacking component of a team; a system or pattern of attack’ in a sport is originally and still chiefly a North American usage. Hmm. Maybe someone can tell me what offensive players are called in other English-speaking lands. Well, maybe some sports lovers would take offence at the notion of calling any players offensive.