p Onto a horse. (defdate|First attested in the mid 20th century.)
a on board | On board; into or within a ship or boat; hence, into or within a railway car. (defdate|First attested from around (1350 to 1470).)(reference-book | last =| first = | authorlink = | coauthors = | editor =Brown, Lesley | others = | title = The Shorter Oxford English Dictionary | origdate = | origyear = 1933| origmonth = | url = | format = | accessdate = | accessyear = | accessmonth = | edition = 5th | date = | year =2003| month = | publisher =Oxford University Press | location =Oxford, UK | language = | id = | doi = | isbn =978-0-19-860575-7 | lccn = | ol = | pages =6| chapter = | chapterurl = | quote =)
a On or onto a horse, etc. (defdate|First attested in the late 19th century.)
a (baseball)on base | On base. (defdate|First attested in the mid 20th century.)
a Into a team, group, or company. (defdate|First attested in the mid 20th century.)
a (nautical)alongside | Alongside. (defdate|First attested from around (1350 to 1470).)
p On board of; onto or into a ship, boat, train, plane. (defdate|First attested around 1350 to 1470.)
p (obsolete) Across; athwart; alongside. (defdate|Attested from the early 16th century until the late 17th century.)