The Mathematical Institute is just off Woodstock Road opposite the Royal Oak (a pub). My office is S3.29 - in the South Wing, follow the obvious staircases upwards to the third floor and turn right, towards where there are several plants in the corridor.
By train - it is about a 20 minute walk from Oxford train station. Head east towards the town centre, along Hythe Bridge Street and George Street, then turn left (northwards) onto Magdalen Street and St Giles, passing the Randolph Hotel and Ashmolean Museum on your left. The Mathematical Institute is on the left hand side shortly after St Giles splits into Woodstock Road (left) and Banbury Road (right) - enter through a courtyard containing a fountain, and the main entrance is at the far right-hand corner. There is a marginally more direct route along Walton Street if you prefer. Beware that Google sometimes directs people to the old Mathematical Institute (now the Department of Statistics) which is on the right hand side of St Giles where it splits into Banbury Road.
By car - it is recommended to park at Pear Tree Park & Ride and take the bus (park&ride 300) which brings you straight in along Woodstock Road and makes a stop at the Radcliffe Observatory Quarter.
By air - There are direct buses (the airline) from Heathrow and Gatwick that terminate at Gloucester Green bus station on George Street. The route from there is essentially the same as from the train station.
Trinity College is on Broad Street, near to the White Horse (a pub) and Blackwells bookshop. The Porter's lodge is through the door to the right of the large blue gates opposite the end of Turl Street. My office is staircase 10, room 7 - head along the main driveway and under the chapel arch to enter Durham Quad with its small octagonal lawn; staircase 10 is at the far right-hand corner.
Directions - it is an easy 10-15 minute walk from Oxford train station. Head east towards the town centre, along Hythe Bridge Street, George Street, and eventually Broad Street. Trinity is on the left at the broadest point of the street.