"一块油画布" can be translated into English as "a piece of canvas for oil painting" or more concisely as "an oil painting canvas" (though the first version is more explicit in indicating its purpose).
Here are examples in sentences:
I need to buy a piece of canvas for oil painting. （我需要买一块油画布。）
The artist prepared an oil painting canvas before starting his work. （那位艺术家在开始创作前准备了一块油画布 。）