Octal to Decimal conversion is obtained by multiplying 8 to the power of base index along with the value at that index position.
(214)8 = 2 * 8v + 1 * 8¹ + 4 * 8⁰
= 128 + 8 + 4 = (140)10

 Converting decimal fraction into octal number is achieved by multiplying the fraction part by 8 everytime and collecting the integer part of the result, unless the result is 1.
0.345*8 = 2.76 2
0.760*8 = 6.08 6
00.08*8 = 0.64 0
0.640*8 = 5.12 5
0.120*8 = 0.96 0
So, (0.345)10 = (0.26050)8

Binary to Decimal conversion is obtained by multiplying 2 to the power of base index along with the value at that index position.
(01011)2 = 0 * 2⁴ + 1 * 2³ + 0 * 2² + 1 * 2¹ + 1 * 2⁰ = 11
(1011)2 = 1 * 2^-1 + 0 * 2^-2 + 1 * 2^-3 + 1 * 2^-4 = 0.6875
So, (01011.1011)2 = (11.6875)10

 Each digit of the octal number is expressed in terms of group of 3 bits. Thus, the binary equivalent of the octal number is obtained.
(24)8 = (010100)2

The binary equivalent is segregated into groups of 3 bits, starting from left. And then for each group, the respective digit is written. Thus, the octal equivalent is obtained.
(110110001010)2 = (6612)