San Giovanni in Oleo is a tiny 16th century chapel or oratory, located just inside the Porta Latina in the rione Celio (the historic rione Campitelli). Pictures of the church at Wikimedia Commons are here. There is an English Wikipedia page here.
The dedication is to St John the Evangelist.
Also see San Giovanni a Porta Latina.
This little chapel, of "St John in Oil", is associated with a strange legend concerning St John the Evangelist, first mentioned by Tertullian in his De Praescriptione Haereticorum c. 36. According to it, the saint was arrested at Ephesus at the end of the 1st century AD, sent to Rome for trial and there put into a cauldron of boiling oil on the orders of the emperor Domitian. He emerged unhurt, whereupon he was forced to drink a cup of poison which did not harm him. After that he was sent into exile on the island of Patmos, where he wrote his Apocalypse.
The legend is demonstrably ancient, but it seems unlikely that it is based on fact -or even that St John the Evangelist was ever in Rome. The method of execution as described is fantastic, but there is a possibility that the legend arose from a memory of an early Christian in Rome who died as a result of being scalded by hot cooking oil during a brawl.
You can see a fresco depicting the legendary cook-off in the church of Santi Nereo e Achilleo nearby.
The poison drinking episode led to his being portrayed in the Middle Ages with a chalice having a little serpent or dragon crawling out of it, and you can see this in one of the frescoes inside.
The origins of the chapel are completely unknown, but many think that there is an older shrine, probably 5th century, below the present chapel. The nearby church of San Giovanni a Porta Latina was certainly founded at the end of this century, although the circumstances there are also unknown, and so the chapel might have been part of the same project.
Further indirect support for the surmise is that the legend is ancient, and so was in circulation by this century. Also the present edifice is octagonal, a common type of building for an early shrine but one that was uncommon in the Renaissance except where an older building was being copied. However no archaeological excavations have been carried out, so the origins are still an open question.
A weighty objection to an early foundation is that the chapel was actually built in the street, and blocked more than half the width of the roadway right up to the 20th century. In the 5th century the area was still urban and had yet to be depopulated, also the street was the ancient Via Latina and hence a major throughfare.
An alternative theory is that the original structure here was a pagan mausoleum erected before the present city walls were built.
The present chapel was built in 1509, and the design was originally ascribed to Donato Bramante. It is now thought that the architect was either Antonio da Sangallo the Younger or Baldassarre Peruzzi. The founder was a French prelate named Benoît Adam who was attached to the court of Pope Julius II as the representative for France at the Sacred Rota.
The work was paid for by Cardinal Francesco Paolucci, and authorized by Pope Alexander VII. Allegedly, the cardinal wanted to appropriate the chapel for his family as a mausoleum -but obviously could not obtain the necessary paperwork from the pope.
There was an emergency restoration in 1967, after the dome fell into disrepair. To relieve the load on it, the original finial was removed and replaced by a plaster copy. The original is now on a plinth in the loggia of San Giovanni a Porta Latina, the Rosminian church nearby which continues to have responsibility for the chapel.
Further repairs had to be carried out around 2012.
The plan of the chapel is based on an irregular octagon, with four long and four short sides.
The exterior walls are rendered in a dull orange, and on each corner is a dark grey stone Doric pilaster following the angle. Rather strangely there are two doors on opposite sides, one facing the city and the other the gate. Such an architectural arrangement was often used for a popular shrine chapel where there would be a queue of visitors, but this chapel has never been one of those.
Above the city door you can see the coat-of-arms of the founder, Benoît Adam, with the inscription Au plaisir de Dieu, and above this a dedicatory inscription on a tablet mentioning him and Pope Julius.
The doorcase has old graffiti cut into the stonework, some dated in the 18th century.
The gate door has an epigraph above it, recording the restoration by Pope Alexander and Cardinal Paolucci. This is topped by stylized mountains surmounted by a star, the arms of the Chigi family to whom the pope belonged.
The pilasters support an entablature with its frieze in the same colour as the main walls, and above this is the very low drum of the dome with the reliefs by Bramante, featuring double roses and palm leaves repeated all the way round. Above the gate door the coat-of-arms of the cardinal is inserted into one of the roses.
The shallow conical dome is tiled, and on top is a large finial featuring what looks like a moschatel flowerhead supported by a bundle of acanthus-leaf scrolls. The flowers are meant to be roses (single, not double), and these and the roses on the drum are derived from the one on the cardinal's shield.
The interior contains a simple free-standing altar, and a large fresco on each of five of the walls. The other three walls have the two doors and the only window which faces away from the street. Over the city door is a tablet with a medieval poem which reads:
Martyrii calicem bibit, hic atleta Iohannes, principii verbum cernere qui meruit, verberat hic fuste proconsul forfice tondit, quae fervens oleum laedere invaluit. Caeditur hic olium dolium cruor atque capilli, quae consecrantur, inclita Roma, tibi.
("This athlete John drank the cup of martyrdom, who deserved to understand the words of the First Principle. Here the proconsul beat him with a rod and scraped him with pincers, whom the boiling oil was unable to harm. [His] flesh and the hairs on his head were sacrificed in this oil jar, which are consecrated to you, glorious Rome")
The reference to the First Principle concerns the first chapter of his gospel.
The scenes depicted in the frescoes are: St John Condemned to Death, Being Led to his Execution, Being Lowered into the Cauldron, Being Forced to Drink Poison from a Cup and In Exile on Patmos.
The stucco work and the paintings with scenes from the life of St John the Evangelist were executed by Lazzaro Baldi, but the former was restored in 1716 and it is not now possible to ascribe it.
The chapel is kept locked. Until recently, guidebooks advised you to ask for the key at the Rosminian College, the large pink building on the right just down the road. However, the brethren are not nowadays likely to hand the key over to any stranger turning up on their doorstep. What you can do is to ask about access arrangements.
It seems that the easiest way to obtain entry is through a guided tour of the basilica, which would include the chapel. The brethren should have details of any proposed tours.
The legend described above used to have its own feast-day in the Church's general calendar, "St John at the Latin Gate" on May 6th. This was suppressed in 1970, but it is theoretically possible for you to find Mass being said in the Extraordinary Form in the chapel on that date.
However, the Diocese does not now list the chapel as a place where Mass is said publicly. It has been referred to as a sacello or devotional shrine rather than as a cappella, which may hint that it is not now an approved place for saying public Masses.
In fact, the original dedicatory inscription refers to it as a sacello so perhaps it was not even conceived of as a chapel in the first place.
On the other hand, the presence of an altar indicates that it is now regarded as a detached side chapel of the basilica. So, if you are a priest interested in saying Mass here privately it is worthwhile consulting the Rosminian brethren at their convent.
(The chapel has no official diocesan web-page.)
Architectural article (Bad link)