SM53, originally designed MBO and colloquially known as Høka, were a class of 58 trams and 50 trailers built by Høka and Hägglund for Oslo Sporveier. The units were used on the Norway's Oslo Tramway from 1952 until 2000. The 14.7-meter (48 ft 2+3⁄4 in) long and 2.5-meter (8 ft 2+7⁄16 in) wide trams weighed 16.9 tonnes (16.6 long tons; 18.6 short tons). They had four motors providing a combined power output of 180 kilowatts (240 hp), allowing for a top speed of 60 kilometers per hour (37 mph).

The first series of thirty trams in 1950, with delivery in 1952 and 1953. These were designated MBO50. The next order was for new bodies for used Class SS units. The eight motor units were designated MO and nicknamed Chickens, while the twelve trailers were designated TO. These twin-axle units proved unsuccessful and Oslo Sporveier therefore took deliver of more MBO units. The next batch of twelve MBO55 units were delivered in 1957 and the final batch of eight were designated MBO56 and delivered in 1958. These were built for use on the Lambertseter Line, but were found unsuitable for use on light rail service and later moved back to the streets. From 1985 to 1991 eleven units were rebuilt with new cabs, interior and fronts, and designated SM83. Retirement of the class started in 1980 with Chickens. The series were gradually replaced, with the last SM53 units being taken out of service in 1997. The last SM83 were taken out of use in 2000.

At the end of the Second World War Oslo Sporveier had a fleet of 331 trams, of which only the 46 Gullfisk trams were modern bogie-constructions. The remaining were twin-axle and limited in capacity. Oslo Sporveier needed more and newer matériel and considered several options. More Gullfisk were considered, but their aluminum bodies were found to be unsuitable, they had issues with cracks in the bogies and their electrical system was prone to faults. They were design with entrance through the middle door, which had during the high usage during the war proven to be inefficient and often led to two conductors being needed for each car. By then flow-through units with a stationary conductor had become common. There were also significant developments within controllers and 300-volt motors. As such additional orders for Gullfisk were discarded.

An invitation for tenders was issued in 1947. Ateliers de Constructions Electriques de Charleroi offered to build a European adoption of the PCC streetcar, although the necessary licenses for Scandinavia were held by Svenska Järnvägsverkstäderna. The trams met all of Oslo Sporveier's demands, except their high power usage. However, the downside was the high price and that foreign production would require currency exchange licenses, which could be complicated because part of the price was for royalties.

The second offer came from Strømmens Værksted and Norsk Elektrisk & Brown Boveri, for an enlarged model of the Class 1947 being built for the Bergen Tramway. It was a variant of a standardized tram built by Brown, Boveri & Cie. for the Swiss tram market. The final offer was from Hägglund and Høka for a variant of Hägglund's Mustang. These were in various versions being delivered to the tramways in Stockholm, Gothenburg and Malmö.

Oslo Sporveier opted for Høka and Hägglund's bid and in 1950 signed an agreement to buy thirty motorized units with delivery in 1952, at 250,000 Norwegian krone per unit. Høka held the Norwegian rights to a Czechoslovak patent for wooden-filled steel profiles that Hägglund used in its design. The bodies were therefore built by Høka in Hønefoss, while Hägglund built the electrical and mechanical components. Final assembly was carried out by Hägglund and Oslo Sporveier at Sagene Depot in Oslo. The first tram arrived in Oslo on 1 September 1952 and, after final assembly and tests, entered revenue service on 11 November 1952.

The trams were first used on Line 13, which ran from the Sagene Line via Stortorvet to the Kjelsås Line. Half the original series was delivered by April 1953, allowing other lines to also use the new trams. The final tram of the original delivery series entered service on 14 December 1953. Upon delivery the trams had some issues with the controller. Upon occasion they would move to full speed or full brake without being activated by the motorman, causing a few minor accidents. It turned out that this was caused by a combination of mechanical weakness and user errors.

The original plan was to copy a concept from Stockholm, where the trams were run fast and frequently without trailers. However, Oslo Sporveier soon decided to prioritize operating costs and instead run trams and trailers. At first the company operated Class SS trailers, but these proved unsuitable in combination with the MBOs. Oslo Sporveier was not willing to pay the price for new trailers and therefore chose a combination whereby new bodies were built atop existing undercarriages. Two such contractions were built at Sagene Depot in 1953, designated TO52 and the first unit came into revenue service on 23 November. A major issue with the trailers were doors which would not shut properly, often resulting with the trams running with open doors.