I'm surprised that Oliver! (1968) was rated G. It doesn't look like a G rated movie. It contains some strong violence and quite a bit of Drinking. It looks more like a PG film. So why was it rated G? What was the MPAA's justification when they rated this Movie G? This movie would never be rated G by today's standards.
There was no "PG" in 1968, in either the US or UK.
US had G [General] or M [Mature] & no-one in their right mind would edit a family musical to get an M rating, so presumably they were well within the guidelines for the time.
In the UK in 1968 the options were U [Universal] or A [Adult] & as it was a British film, these would have been the specific guidelines it was probably made to.
A didn't even stand for 'advisory' until 1970, so the distinctions were far more broad. PG was introduced in 1972, US & 1982 UK.
Wikipedia - Motion Picture Association of America film rating system &
British Board of Film Classification - History of the age ratings symbols
Because there was no PG when Oliver! was released. As per the Wikipedia article:
The ratings used from 1968 to 1970 were: †1 †2
Rated G: Suggested for General Audiences
Rated M: Suggested for Mature Audiences – parental discretion advised
Rated R: Restricted – persons under 16 not admitted, unless accompanied by parent or adult guardian.
Rated X: Persons Under 16 Not Admitted
While the film might be on the stronger side of a G, it probably wasn't harsh enough to warrant an M rating.
†1: Kennedy, Matthew (2014). Roadshow!: The Fall of Film Musicals in the 1960s. OUP USA. p. 183. ISBN 9780199925674.
†2: Life Magazine. May 30, 1969. p. 55.